Unresolved issues in the latest round of DNA testing in the Hank Skinner case

DNA testing in the Hank Skinner case began again in 2012 after the state of Texas dropped its objections after a fight that lasted over a decade.  After the release of some preliminary data, the prosecution declared that it confirmed Skinner’s guilt, as reported by Jordan Smith in the Austin Chronicle.  Mr. Skinner’s attorney issued a response stating that not all testing was complete and that defense experts had not had a chance to examine the results yet.  Skinner had been found guilty of murdering his live-in girlfriend Twila Busby and her two grown sons.  He claims that he is innocent, and there is an alternate suspect in this case, Robert Donnell, who was Twila’s uncle.

Mr. Skinner’s DNA was found on 10 of 40 items tested.  Many of the items tested appear to have been blood, and they come from various locations around the house.  There are profiles that belong to an unknown individual or individuals, including one unknown profile on a knife that has DNA deposited by Mr. Skinner and by Elwin Caler, one of Ms. Busby’s sons.  There is also an unknown profile from a bloodstain in the sons’ bedroom.  The unknown profiles may not be complete, but that is one of several issues that further testing might resolve.

My conclusions at this time are that the advisory paints an incomplete (therefore misleading) picture of the case and is overinterpreting both the previous DNA results and the present ones.  For example, the prosecution’s advisory summarized the testimony of a witness who claimed that Mr. Skinner threatened her, but the witness later recanted, a fact that was ignored.  The advisory failed to mention anything about two bloody handprints and bloody bootprints that have been discussed in one report on this case. "The State tested the bloody handprints against those of Hank Skinner.  Hank was good for the three handprints near the back of the house.  The handprint on the trash bag, and presumably the handprint on the front storm door, belonged to someone else." The existence of the jacket, the handprints that apparently do not match, and the bloody bootprints that are not his size are some of the objections to the prosecution's case.

With respect to DNA profiling the first problem is that the most compelling piece of exculpatory evidence, what one person identified as Donnell’s jacket, has been lost before it could be tested.  The jacket is stained with blood and sweat.  If either Donnell’s DNA were found on the jacket or if some of the blood were Twila’s), that would establish reasonable doubt, at the very least.  However, at present there is no reference profile of Robert Donnell (see also below).  The failure to obtain his profile could be a consequence of the investigatory tunnel vision that has plagued this case, as a previous entry here indicated.  I suggest that the State of Texas exhume his body or obtain his DNA in some other way.

A second problem centers around whether Twila was sexually assaulted on the night of her death.  The vaginal DNA profiling was performed by standard autosomal DNA methods, in which DNA deposited by a possible perpetrator can become overwhelmed by the victim’s own DNA.  There is nothing in the public record of Y-STR testing being performed, despite its routine use in sexual assaults.  The unique advantage of Y-STR profiling is that it involves only the Y chromosome, which women do not have; therefore, a male contribution to a mixture can be selectively amplified by the polymerase chain reaction.  Either the laboratory did not do Y-STR testing, or it did but the results were not included in the advisory.  In addition the fingernail clippings either showed Ms. Busby’s DNA, or they did not show any DNA.  Moreover, fingernail scrapings can also be tested with Y-STR forensics, which has marginally higher sensitivity than autosomal testing.  Mr. Skinner’s attorney indicated that the defense has requested additional testing; therefore, it is possible that Y-STR results will be forthcoming.

A third problem concerns the existence of DNA from at least one unknown person.  The evidence with respect to the knife appears inculpatory at first glance, yet it is undisputed that Mr. Skinner was bleeding that night.  It is likely that his contribution to the knife DNA profile arose from his blood, and one obvious possibility is that he cut himself while committing the murders. The defense should raise two alternate hypotheses.  There is chance that his profile arose from casual use of this knife around the house prior to the murder.  In addition, Mr. Skinner was incoherent according to one witness due to a combination of alcohol and codeine; therefore, his DNA could have been deposited by his possibly handling the knife at some point after the murder when he was allegedly stumbling around the house.  It is also undisputed that he was in close proximity to the victims that night; therefore, positing that he handled the knife, although speculative, is not unreasonable. 

The Skeptical Juror wrote, "Fresh blood drops were found on the sidewalk, near the front door of the house where the murders took place.  Those drops were tested.  The DNA from those drops belonged not to Hank Skinner, but to an unidentified male." The most recent DNA profiling also did not test the gauze, even though it may have been used to wipe the knife, which is odd.  I wonder whether this profile was similar to or different from the ones turned up in the most recent round of testing, and I also wonder who the DNA donor(s) is(are).  The former question might be addressed by treating the DNA from the blood drops on the sidewalk as something like a reference profile.  DNA mixtures are sometimes deconvoluted by a process known as sequential unmasking, in which a forensic worker is given information about the reference profiles in a gradual manner, only after completing a relevant portion of the analysis.  Yet even the mere existence of blood from another person calls for an explanation.  Was the blood fresh?  If so, who was bleeding besides Mr. Skinner, Ms. Busby, Mr. Caler, and Mr. Randy Busby, Twila’s other son (all of whose reference profiles are available)?

Finally, a fourth problem is that one of the samples may have been contaminated with some DNA from a laboratory worker.  This kind of event actually happens not infrequently in DNA profiling, but it does mean that the defense needs to scrutinize the negative controls and machine logs to look for other evidence of contamination.  Negative controls will detect large scale contamination that affects multiple items of evidence, but will often not detect sporadic contamination affecting a single sample, as Professor William Thompson has pointed out:  “However, the same processes that cause detectable errors in some cases can cause undetectable errors in others. If DNA from a suspect is accidentally transferred into a ‘blank’ control sample, it is obvious that something is wrong; if the suspect’s DNA is accidentally transferred into an evidentiary sample, the error is not obvious because there is another explanation — i.e., that the suspect is the source of the evidentiary DNA.”

One hopes and expects that future rounds of DNA testing will address some of these shortcomings.  Both the victims’ families and Hank Skinner deserve nothing less than a full accounting of what happened that terrible night.  However, the prosecutor brings charges in the name of the people; therefore, every citizen has some responsibility for what happens in a courtroom, including the times when there is a miscarriage of justice.

Presumptive blood testing in the Chamberlain, Taylor, Lovejoy, and Knox/Sollecito cases

Part 35 in the Knox/Sollecito case

In this post we will examine presumptive testing in the cases against three other individuals before returning to the Knox/Sollecito case.  Presumptive blood testing has been a subject of this series twice before.

Lindy Chamberlain

In 1982 Lindy Chamberlain was tried for the 1980 murder of her daughter Azaria near Ayers Rock in the Northern Territory of Australia.  A presumptive blood test in the Chamberlain’s car was positive:  The ortho-tolidene test is similar to the tetramethylbenzidine (TMB) test in that both rely upon the pseudoperoxidase activity of hemoglobin to produce a color change.

There are at least two problems with taking the positive result as strongly inculpatory evidence.  Problem one is that the ortho-tolidene test is subject to a number of false positives, and copper dust (common in the Chamberlain’s home of Mt. Isa) is one substance that produces false positive in catalytic tests such as this one.  Yet the head of the forensic laboratory downplayed the idea that the ortho-tolidene test gives false positives, claiming that interfering substances gave a different color than blood in this experiment.  When asked whether an experienced biologist could confuse a substance such as copper with blood in this test, he replied, “Not while I’m around.”  One wishes that someone had subjected Ms. Kuhl and her superior to a blind test of their abilities.

Problem two is related to the issue of substrate controls.  Substrate controls are used to check areas nearby a stain to see whether a positive test reaction is related to the stain or not.  Dr. T. Raymond wrote,  “The original trial scientist, Mrs Joy Kuhl obtained positive (ortho-tolidine) presumptive tests for blood in a number of areas in the Torana – including the carpet, bolt hole region, nearside seat hinge, console and items that were purported to have been in the car – a chamois, a towel and a pair of nail scissors. Some of these areas were incidentally inaccessible without the removal (using appropriate screwdriver) of surface panels.”  The inaccessible areas should have been considered substrate controls. The fact that they were negative positive should have given the forensic team pause but did not.  With respect to the luminol presumptive test for blood, Barni and coworkers wrote, “Regardless of how the stains are collected, the essential requirements to be met are the recovery of the available blood, the collection of a control sample in a tested area not exhibiting chemiluminescence and the complete drying of the support used for blood collection…”  Mrs. Kuhl also devised a test for fetal hemoglobin (Hb F).  The scientific questions surrounding the Hb F test are outside the scope of the present blog entry.

Gregory Taylor

 Mike Klinkosum wrote an article about the Gregory Taylor case in North Carolina, and how a number of cases of problematic blood forensics were uncovered in an audit. "What was not disclosed at Taylor’s trial was that NCSBI Agent Deaver had only provided the positive results from a presumptive test for blood known as a phenolphthalein [Kastle-Meyer] test. He had withheld from his official laboratory report the fact that he performed a confirmatory test (called a Takayama test) on the two stains that returned negative results for blood as to both substances...Agent Deaver had performed a third test on one of the suspected stains, a species origin determination test (referred to as the Ouchterlony test), designed to determine if the stain was human blood. He received a negative reaction on that test as well, but withheld the results of the Ouchterlony test from his official laboratory report."

Amy Driver reported on the aftermath of the Taylor exoneration. "Jill Spriggs, Chief of the Bureau of Forensic Sciences for the California Department of Justice and the President-Elect of ASCLD, responded [to a question at a hearing of the North Carolina state legislature] 'That is an accurate statement. A lot of times you got no results. It didn’t mean it wasn’t blood; it meant you didn’t have enough sample, or maybe the sample was old. …What else is red-brown that will give you a positive presumptive test for blood? There’s nothing that I know.'" This is a remarkable statement. Plant peroxidases are one source of false positive reactions, and rust is another.  With respect to most crimes, animal blood is also a false positive.

Mr. Klinkosum summarized four categories of wrongdoing with respect to how tests for the presence of blood were reported. He quoted the Swecker-Wolf report on the fourth category:  "The fourth and most serious category involves cases in which the reported actual results of the confirmatory tests were over-reported or not reflective of the results contained in the lab notes. There were five such cases in this category, all handled by [Special Agent Duane] Deaver.3 One of these cases involved a defendant who was executed. In two instances, the words 'revealed the presence of blood' were used when in fact the results of the confirmatory test were reflected in the notes as negative. This language was only used by Analysts when the presence of blood was confirmed by a positive confirmatory test. In three instances the report stated that further tests were 'inconclusive' or 'failed to give any result' when the lab notes reflect negative results. It should be noted that the Analyst, SA Deaver, advised reviewers that he was trained that confirmatory tests had only two possible results, negative or positive. SA Deaver’s lab files, however, revealed these two instances in which SA Deaver used the words ‘inconclusive’ in connection with Takayama test results despite his notes reflecting a negative result in one cases (sic) and three tests and three negative results in other cases."

The problems at North Carolina’s SBI laboratory go beyond one special agent, however. The Raleigh News and Observer reported, “Jed Taub, a 30-year veteran of the SBI, said, ’We didn't report the negative result of a confirmatory test because, really, it's misleading,’ said Taub, who now works as a forensic investigator for the Pitt County Sheriff's Office. ‘We couldn't be sure it wasn't blood, so those tests really didn't matter.’’ One might argue that Taylor's case is a different situation from the Knox/Sollecito case discussed more fully below. In the former case, a negative confirmatory result was withheld. In the latter case, the TMB results were withheld until late in the first trial. However, the more general principle in question is whether or not forensic scientists should disclose all test results and let the chips fall where they may.

In one of an award-winning series of articles at the Raleigh News and Observer, Joseph Neff and Mandy Locke wrote, “The phrasing of State Bureau of Investigation lab reports from the 1980s to 2003 caused a lot of ruckus in 2010, raising the question of whether North Carolina's practices that helped prosecutions were common to crime labs across the country.”  The executive summary of the 2009 National Academy of Science report “Strengthening Forensic Science in the United States: A Path Forward” noted on p. S-15, “The terminology used in reporting and testifying about the results of forensic science investigations must be standardized.” It is unfortunate that the NC SBI laboratory did not adhere to this principle in the Taylor case.  "Regardless of how everyone else did it 20 years ago, the current and best practice is to report the results of all tests, just as the [North Carolina] SBI lab does now," [SBI Director Greg] McLeod said.  Director McLeod’s comments are suggestive of an improved culture at the SBI lab.

Laurence Lovejoy

The Laurence Lovejoy case from Illinois involves the lack of disclosure of a presumptive test. The Illinois State Supreme Court Granted Mr. Lovejoy a new trial. "Despite the positive LCV [leucocrystal violet] test, Camp tested the swab with TMB, a more sensitive and more reliable presumptive test for the presence of blood, as her protocol dictates. The TMB test was negative, indicating that the substance swabbed was not blood." The opinion goes on to state, "The parties do not dispute the facts giving rise to the alleged discovery violation. Both sides agree that Camp did not include her finding that the TMB test produced a false negative, and did not state her reason for the negative TMB test in her report."

The opinion also stated, "Further, if defendant were made aware of Camp’s conclusion that the TMB test produced a false negative because the LCV used all the hemoglobin in the blood, he could have called an expert to refute this contention during his case in chief, or could have chosen to pursue a different line of defense altogether. Defendant was prejudiced when Camp’s conclusions were revealed for the first time at trial." Earlier in the opinion it was made clear that such an expert indeed existed: "In that motion, defendant stated that Dr. Karl Reich, who holds a degree in molecular biology from the University of California, Los Angeles and Harvard University, was prepared to testify that the TMB test used by Camp is the most sensitive presumptive blood test available; that a negative TMB test strongly suggests that there is no blood in the area tested; and that Camp’s testimony that the LCV consumed the reactive blood components, thus confounding the TMB test, was incorrect."  In other words, not knowing that there was a negative TMB test nor knowing what rationale would be offered for its being negative hampered the ability to present a robust defense.

Amanda Knox and Raffaele Sollecito

The luminol-positive areas in the Knox/Sollecito case include two blobs in Filomena’s room, three bare feet in Amanda’s room, three bare feet in the corridor, and one shoe print in the corridor.  The two amorphous shapes in Filomena’s room and the shoe print have Meredith and Amanda’s DNA profile, the three prints in Amanda’s room have her profile, and the three bare prints in the corridor have no DNA profile.

A number of forensic references recommend the use of a colorimetric reagent such as TMB on areas that are luminol- or fluorescein-positive. Lisa A. Gefrides and Katherine E. Welch wrote,  “Either luminol or fluorescein can be sprayed onto large surfaces such as walls or floors and the positive areas areas marked for further testing. Both tests are very sensitive and will indicate bloodstains that may not be visible…One disadvantage to these tests is that both can have false positive reactions. Luminol and fluorescein will react with the same false positives as PH [phenolphthalein] and also with bleach and other cleaning fluids, which may interfere with blood detection on surfaces that have been cleaned. For this reason fluorescein or luminol positive areas should be retested with one of the color change presumptive tests.”  (“Serology and DNA” in “The Forensic Laboratory Handbook Procedures and Practice,” Humana Press, 2006) Stuart H. James and William G. Eckert wrote, “After any positive reaction with luminol, the stained area should be circled with a grease pencil or other suitable marker. The stained area should be checked again with another reagent for blood such as tetramethybenzidine (TMB), phenolphthalein, or o-tolidine.”  (p. 162, “Interpretation of Bloodstain Evidence at Crime Scenes,” 2^nd ed., CRC Press, 1998)

A technical bulletin on Lightning Luminol said, “Luminol is only a presumptive test and should be used in conjunction with a second field presumptive test and followed by laboratory analysis if sufficient amounts of staining are detected…A second presumptive field test should be used on the areas that were detected positive with Luminol. Results should not be reported as a positive presence for blood, only positive indications of blood. It is recommended that laboratory testing, if possible, be followed on staining areas detected at scenes.”  In “The forensic luminol test for blood: unwanted interference and the effect on subsequent analysis,” Anders Nilsson of the Swedish National Laboratory of Forensic Science wrote, “Due to the lower selectivity of the luminol test positive reactions should be verified by other presumptive blood tests like the phenolphthalein test [James, S.H., Kish, P.E. and Sutton, T.P., Principle of bloodstain pattern analysis: theory and practice, 2005 CRC press Taylor & Francis Group, ISBN 978-1-8493-2014-9].”  Although one group of workers (“Accuracy, Reliability, and Safety of Luminol in Bloodstain Investigation,” Canadian Society of Forensic Science Journal 35 (3), September 2002, 113-121) has suggested putting an end to the practice of following luminol with a colorimetric test, it would seem that many laboratories do use the TMB or the Kastle-Meyer test on luminol-positive areas. 

The fact that some or all of the luminol-positive areas had been tested using TMB with negative results did not emerge until Sarah Gino’s testimony in September of 2009, near the end of the first trial. The Massei report stated, “With respect to the Luminol-positive traces found in Romanelli's room, in Knox's room and in the corridor, she stated that by analysing the SAL cards ‘we learn, in contradiction to what was presented in the technical report deposited by the Scientific Police, and also to what was said in Court, that not only was the Luminol test performed on these traces, but also the generic diagnosis for the presence of blood, using tetramethylbenzidine...and this test...gave a negative result on all the items of evidence from which it was possible to obtain a genetic profile…’” (pp. 256-257, English translation)

There has been much discussion about whether or not the luminol-positive evidence items in the Knox/Sollecito case are blood.  I will outline six reasons to be skeptical of the notion that the greater sensitivity of luminol is the explanation for why the TMB test was negative for luminol-positive areas in this case:

There are substances in some of the photographs of the application of luminol that give off a blue glow, as posted in these threads. There were blue flecks on a ruler, on a pair of boots worn by someone within the forensic police, and on the grouting between tiles.  The technical bulletin for Luminol Lightning noted that, “Luminol can give a low grade reaction with some carpet materials.” False positive reactions are seen with many substances besides bleach.

The technical notes for Luminol Lightning caution, “Also, tracking through an area that has been sprayed with Luminol will produce brighter shoe tracks. These tracks can be transferred to other parts of a crime scene. Care should be taken not misinterpret these reactions.”  This might explain the luminol-positive areas in Filomena’s room; however, whether the hallway or Filomena’s room were sprayed first is unknown.  Alternatively, one can speculate that the luminol-positive areas in Filomena’s room were the result of foot traffic carrying an unknown substance between 2 November and 18 December, when the luminol was applied.

Ranges of values for the sensitivities for both the luminol test and the TMB test have been reported in the forensic literature. Some values suggest that luminol test slightly more sensitive, but I have seen claims that both tests can detect blood that is diluted up to 1 part in 1,000,000. However, let us assume for the purposes of illustration that TMB detects blood to 1 part in 10,000 and that luminol detects blood to one part in 100,000, a tenfold difference in sensitivity. However, there is a 10,000 fold range (from undiluted blood to blood that has been diluted ten thousand fold) in which both will be positive. In other words each and every dilution would have to be between 10,000- and 100,000-fold to be TMB-negative and luminol-positive in this hypothetical situation.

Some of the luminol-positive areas were also negative for DNA.  Luminol has a slight negative effect on how much DNA can be culled from a sample, and this depends on the formulation. However, it is certainly not the case that the use of luminol precludes testing for DNA. Moreover some luminol-positive areas did have Amanda’s DNA, so we know that luminol does not always interfere with DNA profiling. So if the luminol reaction were responding to Meredith’s blood, why is there none of Meredith’s DNA in some items of evidence and no DNA at all in others?

If the only reason why TMB would be negative when luminol was positive is the lesser sensitivity of TMB relative to luminol, then there would be no reason whatsoever to follow a luminol test with a TMB. A negative TMB would not be meaningful, and a positive TMB is still not a true confirmatory test. According to the Massei report (p. 258, English translation), Sarah Gino testified about the use of TMB: “She added that, in her own experience, analyses performed with TMB on traces revealed by Luminol give about even results: 50% negative, 50% positive…” This observation suggests that the purpose of the TMB test in the field is to reduce the number of false positives, as also seems to be true based on the citations above. This interpretation is also consistent with Stefanoni’s testimony about the nature of the TMB test, as discussed in the Hellmann report: “Professor Tagliabracci, specified, without being refuted (hearing of July 18 2009, p. 174), that the tetramethylbenzedine (TMB) test is very sensitive, so much as to give a positive result even with only five red blood cells present. Dr. Stefanoni herself, moreover, clarified (preliminary hearing of October 4 2008) that, while a positive test result could be deceptive due to reactivity of the chemical [evidenziatore] with other substances, a negative result gives certainty that no blood is present.”

As blood is made more dilute, the luminol reaction becomes fainter. There is no evidence to suggest that these luminol reactions were especially faint.

Conclusions

A theme in at least three of these cases is that the forensic personnel have an implicit or explicit belief that a positive result in a presumptive blood test means that blood is really present and a corollary belief that any negative results in subsequent presumptive or confirmatory testing is the result of a failure in the follow-up test.  Putting it another way, the Knox/Sollecito case is not unique with respect to its misinterpreted presumptive tests for blood.  It is an open question whether or not the false beliefs would change if forensic laboratories were made more autonomous from prosecutor’s offices or law enforcement.  This is recommendation 4 in the 2009 National Academy of Science report “Strengthening Forensic Science in the United States: A Path Forward.”  Both the full report and an executive summary are available.

In the Chamberlain case, personnel from the forensic laboratory made statements about presumptive test that were false.  They destroyed plates that involved the HbF testing before the defense had a chance to examine them.  In both the Taylor and Lovejoy cases, the state's failure to disclose the test results was simply wrong.  Likewise in the Knox/Sollecito case it was wrong for Ms. Stefanoni to fail to disclose her negative TMB results prior to the trial.  Even when these results become available to the defense during the trial, this may be too late for the defense to adapt its strategy to the new information.

The time of death in the murder of Meredith Kercher

Part 33 in the Knox/Sollecito case

Introduction

The time of death (TOD) in the murder of Meredith Kercher is a central part of the question of whether Amanda Knox and Raffaele Sollecito participated in her murder.  Ms. Popovic saw Ms. Knox around 8:40 PM acting normally.  If the TOD is between 9 and 10 PM on 1 November 2007, then it is difficult to imagine how they could have imbibed drugs or alcohol, met Rudy and initiated events leading to murder in the 80 minutes that followed.  Let us examine the TOD primarily using the physiology of digestion.  This topic was recently discussed at the pro-guilt website, True Justice for Meredith Kercher, and I am grateful to its author for reminding us of the sensitive nature of this topic.  I am also grateful to the translators of the Massei-Cristiani Report and the Hellmann-Zanetti report.

Meredith’s friends’ descriptions of the meal

As summarized in the English translation of the Massei report (pp. 34-38), Robyn Butterworth was not sure about when the young women ate pizza on 1 November but thought that might have been around 6 PM.  They stopped watching the movie, “The Notebook” long enough to put the dessert, apple crumble, into the oven, according to John Follain's book, "Death in Perugia," p. 54.  Amy Frost thought that the time of the meal was 5:30 or 6 PM.  Sophie Purton thought that the time of the end of the meal was perhaps an hour before leaving, which would mean that they finished desert around 7:45.  According to Candace Dempsey’s book, Murder in Italy, Sophie Purton said that Meredith ate only part of her pizza.

Judicial estimates of Meredith’s time of death

Massei and Cristiani, the professional judges in the first trial, put the time of death as a few minutes after 11:30 PM (p. 382).  Follain (Death in Perugia, p. 344) reported that PM Mignini estimated the time of as between 11:20 and 12 midnight.  A summary of Raffaele’s appeal document suggests a TOD of 9:30-10:00.  Hellmann and Zanetti, the professional judges in the second trial, put the time of death as no later than 10:13 PM.

Gastric emptying times

The contents of Meredith’s stomach had a volume of 500 mL, and her duodenum was empty (Massei, p. 115).  Two time periods are commonly used to measure the first portion of digestion.  The time at which food begins to leave the stomach and enter the duodenum is t(lag).  The time it takes for the stomach to empty by half is t(1/2).  Because Meredith’s duodenum was empty, the more useful number is t(lag), not t(1/2).  The time between the start of Meredith’s last meal and the TOD must be less than t(lag).  In some of his comments Judge Massei seems to be concerned with t(1/2) or perhaps the time it take for the stomach to empty completely.  Yet he also wrote (p. 115), “Dr. Lalli also took into consideration the state of digestion. He stated that solids are ingested into the stomach and are not able to reach the pyloric sphincter until they are reduced to a semi-fluid or fluid consistency; the emptying of the stomach then begins to occur when some of the contents have become sufficiently fluid to reach the pylorus, which happens the third or fourth hour after eating. This is when one can find food material at the level of the duodenum (page 63 of the Lalli report).”  This is significantly longer than the average or median values of t(lag) of which I am aware, and it may be that Judge Massei simply misunderstood Dr. Lalli’s report or did not clearly understand the distinctions among the various measures of gastric emptying.  A website at Colorado State University on the pathophysiology of the digestive system shows the differences among t(lag), t(1/2), and the time for complete emptying.  These values are about 80, 150, and 220 minutes, respectively in this diagram.

A 2003 study by Chen et al. (J. Gastroenterology and Hepatology, 18, 41-46) determined a value of t(lag), namely 81.9 ± 17.4 minutes, with a range of 37.1 to 117.8 minutes.  The authors described the test meal:  “The egg [one yolk and two whites] was ingested with two slices of white bread coated with 7 g of margarine and 8 g of grape jelly, followed by 150mL water.”  A paper on gastric emptying times (Hellmig et al., "Gastric emptying time of fluids and solids in healthy subjects determined by 13C breath tests: influence of age, sex and body mass index" Volume 21, Issue 12, pages 1832–1838, Journal of Gastroenterology and Hepatology, December 2006), was published in the Journal of Gastroenterology and Hepatology, which is peer-reviewed. Other articles cited this paper at least 25 times.  These workers described their test meal:  “After addition of 50 mL of low-fat milk, the egg was scrambled and fried in a pan. The solid test meal was completed by a piece of brown bread (50 g) and butter (20 g).”  They showed that t(lag) for a solid meal did not follow a normal distribution.  The median time was 82 minutes, with the 25% percentile at 66 min. and the 75% percentile at 102 min. Out of 82 subjects (Figure 1D), the longest value was 200 minutes, and the next longest was 170 minutes (each value corresponded to a single individual).

If one uses 6:30 as an estimate for when Meredith began to eat, then 9:50 (200 minutes after 6:30) is a working estimate of the outermost reasonable time at which Meredith was still alive. In fact, the meal may have started earlier than 6:30, given the testimony of Meredith’s friends.  The problem for the prosecution does not get much better if one assumes that Meredith began her meal at 7:45 (when the desert was finished, according to Ms. Purton) and uses 200 minutes as the longest possible t(lag).  This assumption puts the latest reasonable TOD at 10:55 PM.  However, this TOD is far less likely than (for example) 9:05 PM, which is 80 minutes after 7:45.  In other words, there is no possible time for Meredith to have consumed her last meal that favors Judge Massei’s TOD over a TOD between 9 and 10 PM.  

Other indications of Meredith’s time of death

The Hellman-Zanetti report did not attempt to use stomach/duodenum physiology to ascertain the TOD.  However, these judges did use other means to reach a similar conclusion.  Meredith attempted to call her mother around 8:56 but was unsuccessful.  Meredith called her mother once a day; therefore, it would be odd that she did not call back in the period in which she was supposed to have been alone in the prosecution’s scenario.  Later there were two activities in close succession, namely an aborted call at 9:58 to Meredith’s answering machine and one at 10:00 PM to Meredith’s bank, but without the country code, neither of which is a complete call.  Finally at 10:13 there is a GPRS internet connection for 9 seconds.  The summary of Raffaele’s appeal document notes that the tower that handled this interaction can reach both the apartment and the location at which the phones were finally found.  It is difficult to see why Meredith would have initiated any of these three later activities herself.

It is strange that Meredith did not try to call her mother after her attempt at 8:56 did not succeed.  Nor did she call, text, or email anyone else after she arrived home, presumably close to 9 PM.  She did not change into night garments or remove clothing from a washing machine.  The Hellmann court put the TOD at 10:13 at the latest, and yet the evidence suggests that the TOD might have been earlier even without bringing in the physiology of gastric emptying.

Conclusions

The cell phone data suggest that Meredith died no later than 10:13 PM, as Judges Hellmann and Zanetti indicated. It is undisputed that there was activity on Raffaele’s computer at 9:08 PM, and the defense argued that there was activity at 9:26 and that the screensaver log files indicate additional activity.  Even by itself, the lack of material in the duodenum strongly suggests that the time of death was likely to be earlier even than 10:13.  Therefore, there was not enough time for Raffaele and Amanda supposedly to become very messed up from alcohol and/or drugs, to meet Rudy Guede, and to initiate a long series of actions that culminates in murder.  It is likely that Meredith was attacked shortly after returning home, probably between 9 and 9:30.

Hank Skinner, the death penalty, and investigative tunnel vision

Update I, 5 November 2011

The Skeptical Juror wrote, "The hairs found clasped in Twila’s hand, the hairs pronounced by the DA to belong to the killer, turned out to come from a male, maternal relative of Twila. They did not come from Hank Skinner. According to the standard set by the DA before the testing, those hairs exonerated Hank Skinner." Twila's uncle had stalked her the night of her murder. The DNA testing described appears to be mitochondrial DNA forensics (one inherits mitochondrial DNA from one's mother). My tentative interpretation of the mitochondrial DNA is that the data tend toward innocence but are not yet conclusive. As others have said, I am not certain Mr. Skinner is innocent, but I cannot see a persuasive reason not to test the items in question.

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The state of Texas is trying to put Hank Skinner to death for the murder of a family of three people. DNA testing showed that blood smears on his shirt matched two of the victims. Mr. Skinner was undoubtedly at the scene of the crime, but his mental and physical state at the time open the question of whether or not he would have been capable of committing murder. A recent Texas law was designed to apply to this case, to allow further DNA testing.

Radley Balko has been following the case of Hank Skinner for some time. Writing for Reason Balko noted, “In 2000 DNA tests were conducted on blood taken from a roll of gauze and a cassette tape found in the house; that blood didn't match Skinner, his girlfriend, or her sons.” At Huffington Post he wrote, “There is DNA from the crime scene that could exonerate Skinner -- or could affirm his guilt -- that has never been tested. That includes blood from the murder weapon, blood from a jacket left in Busby's home, a rape kit taken from Busby, scrapings from under Busby's fingernails and hairs she was clutching at the time of her death -- hairs that likely came from her killer.”

The State of Texas has argued that since Skinner’s attorney at the time did not press for testing these items. Radley Balko continued, "’They only tested the material they thought would implicate Skinner,’ [Professor David] Protess told me in an interview last year. ‘They fixated on their suspect, and once they thought they had enough for a conviction, they stopped.’” Private investigation several years after the crime turned up a second plausible suspect.

Students of the Knox/Sollecito case should be especially troubled by investigatorial tunnel vision. In the first place, there are good reasons to question the competency of Skinner’s first attorney, apart from his decision not to seek testing. Second, his attorney’s decision was contrary to the wishes of Mr. Skinner, as he expressed in a letter in 1994. Third, the whole issue of whether or not the attorney should have sought testing is misdirection: the investigators should have tested these items as a matter of good forensic science. Here is a link to a petition for full DNA testing.

Some helpful commentary on the acquittal

Part 33 in the Knox/Sollecito case

On 3 October 2011 a court acquitted Amanda Knox and Raffaele Sollecito of the murder of Meredith Kercher. A few articles stood out. Christiana Patterson’s title, “We prefer a crazy story to the truth,” as good a one-sentence summation of the case as any I have seen. She also indicated one of the main problems with the one of the most significant pieces of evidence against Knox and Sollecito: “One piece of ‘evidence’ wasn't ‘found’ for 47 days. It was found, in fact, the day after a TV programme highlighting flaws in the case.” She also empathized with the Kercher family’s dilemma of whether to continue believing that Knox and Sollecito are somehow culpable or to reconsider their understanding of the case.

Of the prosecution’s version of the murder as a sex game gone wrong Deborah Orr said, “An astonishing number of people around the world were keen to believe this scenario, although there is no evidence to support it. Some still are.” Of the Kercher family she concluded, “A lot of people share blame for the mental torture this family have been through. Amanda Knox is not one of them.” Indeed. Nor is Raffaele Sollecito.

Ms. Knox’s seemingly odd behavior led some to suspect her; A friend of mine recalled a video of Ms. Knox as wildly kissing her boyfriend Mr. Sollecito outside of the apartment shared by Ms. Knox, Ms. Kercher, and two other women. Yet Ian Lesie noted, “In fact, the video is anything but sexy. Knox, looking wan and dazed, exchanges chaste kisses with Sollecito, who rubs her arm consolingly.” He saw this case as “a particularly pungent manifestation of a universal trait, one that frequently leads criminal investigators and juries astray: overconfidence in our ability to read someone else's state of mind simply by looking at them.”

With considerable justification Carole Cadwalladr observed, “The whole case, the prosecution's version of events, the media coverage, and the court of public opinion, has been viewed almost solely through the prism of Knox's looks and sexuality.” Was Meredith Kercher shoved aside? To some degree, but so was Ms. Knox’s then boyfriend, Raffaele Sollecito. Most remarkably, Rudy Guede, the sole person whom the evidence indicates sexually assaulted and killed Ms. Kercher has also been largely ignored. In an alternate universe he would have been the person most vilified, and with complete justification.

Update 1 (11 PM)

Dylan Evans saw "confirmation bias" and "imagination inflation" in the investigation: "Unfortunately, law enforcement officials are as ignorant of this phenomenon [imagination inflation] as they are of confirmation bias. When they ask suspects to imagine their possible role in a murder they do not at first remember, therefore, this very exercise may unknowingly lead the suspect to believe that he or she was really involved."

Amanda Knox and Raffaele Sollecito: There is no mixed blood

Part 32 in the Knox/Sollecito case

Introduction
The Conti-Vecchiotti report casts further doubt upon the reliability of the bra clasp and the kitchen knife, two of the strongest, yet seriously flawed, pieces of evidence against Ms. Knox and Mr. Sollecito in this case. With the appeals trial expect to wrap up shortly, it is time to reexamine the strength of the other biological evidence against them, especially the mixed DNA traces. An anonymous pro-guilt blogger did so several months ago. The related question of whether or not the luminol-positive areas (some of which had DNA profiles) were truly blood was the subject of a previous entry here and a discussion at Let’s Talk about True Crime.

Which samples contained mixed DNA?
There are two amorphous, luminol-positive spots in Filomena’s room (Reps. 176 and 177), although the alleles which may be Amanda’s are quite weak. There is a shoe print in the hallway (Rep. 183). In the shared bathroom there is a sample from the bidet drain, the cotton flock container on the floor, and from the inside of the washbasin (Rep. 137). There is no DNA from Amanda in the murder room, either by itself or mixed with Meredith’s DNA.

Did the jury in the first trial find the mixed DNA to be significant?
Barbie Nadeau wrote, "The defense's biggest mistake, according to interviews with jurors after the trial, was doing nothing to refute the mixed-blood evidence beyond noting that it is common to find mingled DNA when two people live in the same house. The jurors needed more than that. 'To have mixed blood, you have to both be bleeding,' one of them remarked to me after the verdict. It was obvious that Meredith was bleeding, but why was Amanda bleeding?'" ("Angel Face," pages 152-153)

Does mixed DNA indicate mixed blood?
The passage quoted above implies that at least one juror thought that the two were equivalent, but this is a fallacy. There are three possibilities with respect to mixed DNA and blood: neither source, one source, or both sources of DNA could be blood. A commonly repeated fallacy of this case is to equate a mixture of DNA with a mixture of blood (to assume possibility three). For example in a recent Dateline broadcast Barbie Nadeau said, “I have to say, I live in a house with 3 people, my two sons and my husband, I guarantee you I have no mixed blood with any of them, anywhere in my house. I don't bleed where they bleed, we don't bleed at the same time. There would never be my mixed blood, their blood and my blood anywhere ever.”

The false equivalence between mixed DNA and mixed blood dates back at least to the publication of the book Darkness Descending. In this book Colonel Luciano Garofano, a retired officer of the Carabinieri, said, “However, here is the electropherogram and you can see that the RFU value is very high, so the sample is undoubtedly blood, which is the body fluid that provides the greatest amount of DNA. In some cases you see higher peaks of Amanda's DNA than Meredith's. Amanda has been bleeding. Nor is it old blood, as the defence might say, because blood decays fast. We have the same result on the cotton-bud box. The light switch was over-scrubbed, but from the film the way the cotton-bud was good enough. There too we have mixed blood. So that's pretty significant for Amanda, Unfortunately for her, she bled at the same time Meredith was bleeding. That's a lot to explain." (Darkness Descending, page 371).

There are several reasons why Garofano’s interpretations are wrong. In response to a question of mine, Professor Dan Krane wrote, “Inferring tissue source from peak heights is just plain silly -- to the point of being absolutely outrageous. It hardly bears more comment than that, but if high peaks mean blood then what would you expect from semen which has a ten to one hundred fold higher concentration of DNA?” Professor Greg Hampikian concurred with the view that peak heights were not an indication of whether or not blood was the source of DNA. The peak heights for Raffaele’s profile on the cigarette butt were reported to me as being about the same height as those on the cotton box, and the former are presumably from saliva.

Colonel Garofano’s claim that the DNA from blood decays quickly is difficult to evaluate. A paper (Park et al., “Direct STR Amplification from Whole Blood and Blood- or Saliva-Spotted FTA without DNA Purification,” J. Forensic Sci., March 2008, Vol. 53, No. 2, 335-41) showed that 1-2 year-old blood samples gave strong signals in DNA profiling when stored in the form of FTA cards (which contain stabilizers); therefore, their study does not exactly refute what Colonel Garofano claimed, but it does not support his claim, either. However, this paper also showed that saliva gave tall peaks in DNA profiling, which is one more indication that peak height cannot be used to infer the biological origin of a sample. The rate of decay of a DNA sample depends upon so many factors that dating DNA by its degradation is not practical. Furthermore, even if one were to accept that DNA peaks from blood did degrade very quickly, one might have to conclude that the luminol-positive, mixed-DNA samples were not blood, inasmuch as the luminol was applied on 18 December, more than a month and a half after the crime.

What did Massei conclude with respect to the mixed DNA samples?
In contrast to the juror quoted above, the Massei report did not assume that mixed DNA was equivalent to mixed blood (pp. 278-279, English Translation). “It should then be highlighted that in that same bathroom various [300] trace specimens were found, of a mixed nature and testing positively for blood. It is true that, according to what was asserted and explained, it is not possible with a mixed trace specimen that tested positive for human blood to determine which of the trace’s contributors the blood belongs to. In this case, however, non-mixed traces were also found, which were shown to be of a haematological nature [i.e. blood] and turn out to have the biological profile of the victim.” The report continued (p. 279), “And it is probable - not necessary, but probable - that during the following act of scrubbing the hands to remove the blood, he/she left the mixed trace consisting of Meredith’s blood and of cells which had been removed by rubbing during the act of washing.”

The Massei motivations report acknowledged that Amanda had no wounds and therefore was not bleeding. It also noted that DNA by itself gives no indication of when it was deposited (see below). In summary Massei thought that the mixed DNA did not necessarily indicate mixed blood, but he believed that the traces were deposited simultaneously, at least partially on the basis of Amanda’s declaring that the bathroom was clean on the afternoon of 1 November. Such a position is problematic in that a clean bathroom does not necessarily imply a DNA-free bathroom. Moreover, there is no reason to rule out Amanda’s depositing the DNA on the morning after the crime in addition to the possibility that she deposited it before the crime.

Does mixed DNA have to be deposited at the same time?
In general the presence of DNA almost never gives an indication of how or when it was deposited. The abstract of an article (“DNA profiling of trace DNA recovered from bedding,” Forensic Science International, Volume 159, Issue 1, 25 May 2006, Pages 21-26) on DNA profiling states in part: “The results indicate that the DNA profile of an individual can be obtained from bedding after one night of sleeping in a bed. The DNA profile of the owner of the bed could also be detected in the foreign bed experiments. Since mixed DNA profiles can be obtained from trace DNA on bedding, caution should be exercised when drawing conclusions from DNA profiling results obtained from such samples.” This is a good example of mixed DNA that could not have been deposited simultaneously.

How common is mixed DNA?
Head of the US National Institute of Standards and Technology's genetics group, “[John] Butler has reviewed more than 5000 DNA samples from 14 US labs and found that mixing is a common occurrence: 34 per cent of the samples he studied included DNA from two people, while 11 per cent were three or four-person mixtures.” Although some fraction of the two-person samples are from the victim and the perpretrator, it is not reasonable to suppose that this is the case for all of them, let alone the three or four-person mixtures.

Are there other mixed DNA samples in this case?
In addition to the mixed DNA of Meredith and Amanda at the girls’ flat, there are also three mixed DNA samples containing Amanda’s and Raffaele’s DNA at his flat. One was found in Sollecito’s bathroom, one was found in his bedroom, and one was found on a pair of rubber gloves. The former two are also luminol-positive, but the identity of the luminol-reactive substance is not known. All three mixed samples are likely to be the result of cohabitation. Amanda’s and Raffaele’s DNA was also found on a cigarette butt at the girls’ flat (p. 193, Massei Report, English translation). The cigarette butt is also interesting in that some of the peaks comprising Amanda’s profile are moderate in intensity, despite possibly being the result of secondary DNA transfer (Amanda does not smoke cigarettes).

Do any of the samples contain DNA from a third party?
Sara Gino’s testimony indicated that in sample 177 in Filomena’s room there were alleles besides those of Meredith and Amanda. I have also examined a copy of the electropherogram. In the D19S433 locus, four alleles are marked: 12, 13, 16, and 16.2, but there are unlabeled alleles at 14 and 15 or 15.2. If one acknowledges that it might have been deposited at some other time than the murder, then one must also acknowledge the same possibility for Amanda's DNA.

Could the forensic team have run controls?
Besides the issue of how samples were collected in general, the forensic police could have done substrate controls, where they examined areas for DNA that were a few inches away from putative blood stains, as materials scientist Dr. Mark Waterbury suggested. If they had found Amanda’s DNA in some of those locations, it would have been suggestive of innocent DNA deposition.

They could also have performed or cited studies of DNA deposition in bathrooms, if such studies existed already (I am not aware of any). There are a number of ways that such studies could be performed. For instance, one could take blood from person A and place it in person B’s bathroom, then collect DNA samples. If one found mixed DNA from A and B, it would strengthen the hypothesis that Amanda deposited DNA in the normal course of everyday living.

Is there precedent for mixed DNA arising through contamination?
Many cases of contamination show a mixture of DNA from the analyst and a potential suspect, as discussed in the previous blog entry. One case, the murder of Jane Mixer, showed contamination from two potential suspects, Gary Leiterman and John Ruelas. However, Ruelas was four years old at the time of the murder and lived in a different city. Therefore, this is probably a case in which both profiles arose from contamination.

Could the way the DNA was sampled have resulted in mixed DNA?
The Massei Motivations report (p. 278, English translation) indicates that the defense thought that the mixed traces were meaningless: “All the more so since the samples had been taken using the same blotting paper which had been used for various parts of the bidet and the sink.” Even Colonel Garofano (a strongly pro-prosecution commentator on the case) was dismayed at the way the washbasin trace was collected, noting, “The fact that the sample was collected by wiping both the edge and the plughole is dangerous. You’re likely to find all sorts of stuff in the plughole.” (p. 370, “Darkness Descending”)

Did the police take every precaution to avoid contamination?
No, there are several ways in which the work could have been improved. Ms. Stefanoni’s view was that liquid samples are liable to cross-contamination, but dry traces are not. In the English translation of the Massei report (p. 203) it says that Stefanoni “specified” that gloves were changed “every time an object was touched that was particularly soaked with blood, and when it was obvious that the gloves would be soiled;” On pages 204-205 she indicated that the presence of a liquid is necessary to bring about contamination by touch.

Ms. Stefanoni’s view is out of the mainstream. On page 38 of John Butler's textbook “Forensic DNA Typing,” he wrote, “Use clean latex gloves for collecting each item of evidence. Gloves should be changed between handling of different items of evidence.” At Forensic Magazine in the article “Evidence Handling and Collection” Dick Warrington wrote, “Go about collecting evidence. I can’t say enough about avoiding cross contamination. Put on gloves, use gloves, change gloves. Do that every time you touch a piece of evidence. Likewise, use disposable tweezers, scalpels, etc. Change these each time they are used, as well.” Warrington also wrote an article for Forensic Magazine called “DNA Collection and Packaging,” that discussed the use of gloves and tweezers to avoid contamination. Orchid Cellmark’s guidelines state, “Use clean latex gloves for collecting each item of evidence. It is recommended the gloves be
changed between the collection of each item of evidence.” If the police handled an item of evidence with Amanda’s DNA then handled an item with Meredith’s DNA, the glove could carry Amanda’s DNA into the other sample.

Are there innocent explanations for the mixed DNA in Filomena’s room?
The luminol work that first identified some of the areas that later were shown to contain mixed DNA traces was performed on December 18, 2007. By this time many police personnel had been in the girls’ flat, and many of Meredith’s items had been tossed about. This raises the possibility that the forensic police tracked the genetic material of either Knox or Kercher into Filomena’s room from the hall. The forensic police who were recorded on 18 December wore one-piece tyvek garments but did not appear to have any outer shoe covering. Former FBI agent Steve Moore noted that they did not change shoe covers going from one room to another that that this creates the potential for cross-contamination. This is especially worrisome in that several members of the team are quite close to the dried bloodstains in Meredith’s room, as can be seen in parts 10 and 11 of a series of videos taken on 18 December 2007.

In addition the luminol-positive spots are only presumptive blood; these tested negative by tetramethylbenzidine, a second type of presumptive test, and there is no record of confirmatory blood testing. Therefore, it is open to debate whether or not the luminol-positive substance is even blood. One photograph of the luminol-positive footprints in the hallway also show blue specks on the ruler and on the boot of one of the forensic police officers. It is unclear what the luminol-positive substance was in this case, or whether it could have contaminated other items of evidence.

Conclusions
Mixed DNA is commonly observed and is not equivalent to mixed blood. In general DNA samples cannot be dated, and any two profiles within a sample may have been deposited at different times. The mixed DNA in the bathroom may have been created by Meredith's blood falling on Amanda's biological matter that was already there. The chances of this happening might have been lessened if the forensic police had taken a smaller trace with respect to the washbasin, for example. Dirty gloves or dropped swabs (which happened elsewhere) made have mixed DNA during collection. The police or the inhabitants of the flat may have tracked Meredith's blood into Filomena's room. The evidentiary value of these mixed DNA samples is very low.

The likelihood of DNA contamination

Part 31 in the Knox/Sollecito case

The subject of DNA contamination has been a frequent topic of this blog. DNA contamination is again near the forefront of the Kercher murder and the second trial of Amanda Knox and Sollecito. The court-appointed, independent experts, Drs. Conti and Vecchiotti, have issued a report that raises the strong possibility of contamination. The odds that a forensic DNA sample are contaminated are hard to determine yet very important. Let us first examine a specific statement made with respect to environmental contamination and this case, then look at the question more generally. A subsequent entry will examine the collection practices of the Rome lab and Conti’s and Vecchiotti’s evaluation of their work in this case.

Professor Giuseppe Novelli, a researcher into medical genetics and forensic DNA profiling, said, «Il contaminante va dimostrato, dove nasce e dove è. Il gancetto contaminato dalla polvere? Più probabile che cada un meteorite e butti giù questo tribunale» "The contaminant needs to be demonstrated, where it comes from and where it is. The clasp contaminated by dust? It's more likely that a meteorite comes down and knocks down this courthouse." (translation by komponisto)

Let us assume that Dr. Novelli is specifically referring to household dust. The potential for dust being an issue in forensic DNA testing first came to light with the publication of a paper from Bonnie Brown and coworkers: Toothman et al., “Characterization of human DNA in environmental samples,” Forensic Science International 178 (2008) 7–15. These workers sampled dust from offices, research laboratories, and classrooms.

Figure 3 in this paper is an electropherogram of dust from a classroom, and it shows peaks of 1000 to 3500 RFU corresponding to 3-6 alleles in some loci associated with shorter pieces of DNA. As the length of the DNA fragments increases (moving from left to right on the elecropherogram), the height of the peaks decreased. The authors point out that this is consistent with DNA template that is partially degraded. The combination of a sample’s being a mixture and its yielding only partial profiles makes it very difficult to identify individual profiles.

The authors wrote, “Results of this study have implications regarding the processing of forensic samples. First, the presence of genotypeable human DNA in dust illustrates a significant
potential contamination source in forensic investigations. Twenty-five of 36 samples contained sufficient input human DNA for STR analysis using the AmpFlSTR® Profiler PlusTM assay (~1.0 ng), and 36% (including low-input samples) produced alleles at multiple loci. These results demonstrate that even though anti-contamination measures may be in place at a crime scene and the laboratory, trace DNA derived from dust in the vicinity of other evidence is capable of producing signals higher than background noise in STR analyses.”

Trace DNA and the environment
Secondary transfer is the movement of DNA from a donor to an intermediate object then to another object from which it is collected. Some forensic scientists only classify it as contamination when it occurs subsequently to the object’s being taken into custody; however, other workers prefer to treat innocent secondary transfer as equivalent to contamination on the basis that neither is relevant to the investigation. Both secondary transfer prior to an object’s being collected and contamination need to be understood more thoroughly for trace DNA to be used routinely in DNA forensics. Van Oorshot and colleagues wrote, “Greater effort needs to be made by police/crime investigators to investigate how a DNA sample arrived at the location where it was found, as well as by scientists to better understand the impact of activities on the relative amounts of DNA from particular sources at a crime scene… Some preliminary contributions to our knowledge of transfer in relation to residential burglary and street robbery have recently been made [67].

In their review of trace DNA in forensics Van Oorshot and colleagues suggested six remedies to address the problem of contamination:
“1. perform more studies similar to those of Raymond et al. [67], Cook and Dixon [202], Dowlman et al. [203] and Toothman et al. [201] in order to learn more about the occurrence and persistence of DNA on particular surfaces in different environmental conditions” Understanding how prevalent and persistent background DNA is in the environment is far from being completely understood. Dr. Novelli declined to comment for this report, but his comparison with a meteorite is premature, at best.

The relationship between DNA sample size and the ease of transfer
In their 2010 review article Van Oorshot and colleagues wrote, “Contaminant DNA may appear as either the major or minor sample within a mixture or, alternatively, may overwhelm the target DNA completely.” One explanation for this somewhat counterintuitive statement is that contamination in the laboratory may introduce DNA from previous PCR amplifications by a number of possible routes. However, the two main DNA profiles of interest (Meredith’s on the knife blade and Raffaele’s putative profile on the bra clasp) both involve relatively small amounts of DNA; therefore, we will focus on small samples.

The discussion of low copy number (LCN) testing from the Crown Prosecution Service noted, “This increased sensitivity means ultra-clean laboratories are needed for the testing to minimise contamination of the sample by DNA from any other source.” The New Zealand Institute of Environmental Science and Research has spent $1 million building anticontamination areas for low copy number (LCN) DNA forensics. The New Zealand Herald wrote, “The bogey is contamination. The very sensitivity of the technique which enables it to extract a DNA profile from the tiniest sample also makes it extremely vulnerable to contamination. Stringent measures are needed to minimise that risk… We live in a ‘soup’ of DNA, explains ESR forensic programme manager Keith Bedford. ‘If I were to shed dandruff, massive amounts of dna could fall ... hair could carry DNA. The way I am speaking at the moment, we could probably detect DNA on this pad in front of me.’”

Sara Gino testified for the defense in the trial of the first instance, and some of what she had to say is pertinent to this issue. From the Massei report (p. 258, English translation): “She reaffirmed that [the risk of] contamination exists, and emphasised that in minimal quantities of DNA there is not necessarily a greater risk of contamination but it was easier to notice the effects of the contamination and be misled (‘...It's not that the risk of contamination is greater; but it is easier to see the contamination...’ page 92).” In response to a question on this subject, Professor Dan Krane responded, “There is absolutely no question but that contamination is a much greater problem in LCN cases than conventional DNA testing. The reasons that it is a greater problem are both because it is easier to detect contaminants ([Sara] Gino's point) and because it is easier to transfer (and to transfer without knowing) smaller amounts of DNA than larger amounts of DNA.”

Some examples of DNA contamination
Farah Jama was a young man accused of rape on the basis of his DNA seemingly being found on the alleged victim. Mr. Jama is black and at 21 was too young to have entered the club at which the incident occurred, which catered to people over 28. Moreover, the alleged victim did not recall seeing a black man at the club that night. Yet, as Milanda Rout wrote, “But the judge and the jury did not buy his alibi, despite supporting evidence from his father, brother and friend. Instead, they believed the forensic scientist who testified there was a one in 800 billion chance that the DNA belonged to someone other than the accused man.” After Mr. Jama spent more that eighteen months in prison, he was released because prosecutors said that they could not rule out contamination. The contamination event may have occurred during two forensic medical examinations, one of the victim and the other of Mr. Jama on an unrelated matter that occurred one day earlier.

Perhaps the most thoroughly studied case of contamination is that which occurred in the Jaidyn Leskie case. This blog has covered the Leskie case on two previous occasions. The DNA of a woman who probably never left her village was found on the clothing of the submerged body of a toddler. She was a mentally challenged woman who may have been raped, which is why her DNA was being examined. Because the woman was such an exceedingly unlikely suspect, the only reasonable explanation was contamination. Contamination has been documented on several occasions when evidence items from unrelated cases are examined within a few days in the same lab.

Russell John Gesah was charged with the rape and murder of a mother and child. Kathleen Skeen wrote, “A Victorian Police Forensic Services Centre review found clothing with Mr Gesah's DNA from an unrelated offence had been examined on the same day and same surface as clothing from the Tapp case." The Gesah case, and the murders of Jane Mixer and Jane Durrua (see below) are all examples of DNA cold hits. The Gesah case prompted the State of Victoria to reexamine thousands of cases (see below).

Gregory Turner might have been convicted of murder on the basis of DNA evidence. However, a forensic worker contaminated a key piece of evidence with his and her DNA. She also acknowledged contamination in two other cases on which she had worked. An interesting aspect of the Turner case is that the DNA from the victim came from her fingernails, and Mr. Turner’s DNA came from his wedding ring. These facts suggest that the presence of liquids is not necessary to bring about cross-contamination, in contrast to the implications of Patrizia Stefanoni’s testimony in the present case.

The murder of Jane Mixer was initially attributed to a serial killer. When Gary Leiterman’s DNA was found on the decades-old evidence, he was convicted. However, the presence of the DNA matching then four-year old John Ruelas on the same item of evidence (despite Ruelas’s living in another city) strongly points to this being another example of contamination. This illustrates another important principle. One does not always know the precise moment that contamination occurred, but one can infer contamination when the direct deposit of DNA is shown to be highly unlikely.

A seemingly solved cold case that turned out to be contamination involved the 1968 murder of Jane Durrua. Jerry Lee Bellamy’s DNA was found when the evidence was tested in 1999. Evidence against Mr. Bellamy in an unrelated case was tested on the same day as items from the Durrua case. The actual evidence of contamination was not conclusive, but despite this, charges against Mr. Bellamy were dropped. Alleged serial killer Robert Zarinski was later arrested, but he died before he could be tried.

The difficulties in quantifying the frequency of DNA contamination
Not all labs document contamination events. Some labs argue that contamination that is identified with the use of negative control experiments does not count as contamination. Negative controls will spot wholesale contamination events but will not necessarily catch sporadic contamination. These facts make it difficult to quantify how frequently contamination occurs. However, it does not seem to be an especially rare event. Professor Thompson is a lawyer who specializes in probability theory as it relates to DNA profiling. Maura Dolan reported that he is among the leading authorities on laboratory errors in the United States. In response to a request from the Los Angeles Times to review the records from some California forensics labs, Thompson said, “’on a regular basis, laboratory personnel make mistakes that could lead to false identifications’ of suspects.” He also indicated that what has emerged in recent years is just “the tip of the iceberg.”

In 2008 Professor Thompson wrote an article, “The Potential for Error in Forensic DNA Testing (and How That Complicates the Use of DNA Databases for Criminal Identification"). “Doubt was also cast on a number of convictions in Queensland when a forensic scientist who had previously worked for a state forensic laboratory publicly expressed concerns about the reliability of the lab’s work. He told The Australian newspaper that it was not uncommon for the lab to mix up DNA samples from different cases.[62] For example, he said that analysts’ own DNA, from blood samples used as analytical controls, often was mixed up with (or found its way into) casework samples, creating false matches: “[Q]uite often my (colleague) would walk down the aisle and say, ‘I’ve just committed another rape on the Gold Coast.’”[62] The analyst said that while many such errors were caught, sample limitations made it impossible to resample or retest in some questionable cases.” These remarks underscore the notion that DNA contaminations are not a rare event.
[62. A. McDonald, “DNA evidence claim clouds Australian convictions,” The Australian, July 8, 2006.]

In response to the Russell John Gesah contamination incident (see above), the Victorian police reexamined their cases involving DNA forensics. During the period from 1988 to 2008 the Victorian police service handled 7000 cases involving DNA. According to Peter Gregory and coauthors, “In 2003, Mr Scheffer told an inquest on Moe toddler Jaidyn Leskie that since late 1999, 39 cases had been identified as requiring "diagnostic and corrective action", with most involving contamination.

Finally, testimony reported by Annabelle McDonald (in The Australian) implied that mixing up samples is a not uncommon event. Although mislabeling of samples (if that is what mixing up means) is not itself contamination, it has the potential to lead to the same erroneous judicial result. A mislabeling in Nevada was uncovered during an independent review of the Lazaro Sotolusson case. In addition Dwayne Jackson was also the victim of a similar mistake at the Las Vegas forensics lab.

Conclusions
There is not yet enough information on environmental contamination to make conclusive statements about how common environmental contamination is; however, DNA is deposited routinely in all sorts of ways that are unrelated to criminal activity. The authors of a recent study believe that environmental dust can give rise to extra alleles in evidence samples. The frequency of contamination is difficult to quantify, but it is not an especially rare occurrence. The chances of contamination are greater for DNA in the low template range than they are for larger samples. Historical examples of contamination suggest that it is more likely to occur when items of evidence are processed closely in time. Contrary to the implication of Dr. Novelli's remarks, it is rarely the case that the exact mechanism of contamination is proven.