How Scarily Sloppy DNA Evidence Convicted, Then Released, Amanda Knox

By Veronique Greenwood | October 13, 2011 2:59 pm

You wouldn’t know it from crime shows, but DNA evidence can have more holes in it than Swiss cheese. And the Amanda Knox murder appeal, in which the DNA evidence that led to Knox and former-boyfriend Sollecito’s convictions in 2009 was thrown out, provided a great example of how a powerful tool, when used incompetently, can be worse than useless.

Getting a DNA fingerprint—matching a particular piece of DNA to a specific individual—is a very delicate process. Samples from the crime scene must be taken immediately, and the object they’re found on must be pristine, not touched by anyone else for quite a while. John Timmer at Ars Technica deftly explains the science behind DNA fingerprinting  and asks experts how the DNA evidence in the Knox trial, taken from the victim’s bra clasp and a knife, measures up. It ain’t pretty:

The experts were not kind to the evidence. The bra clasp, it turned out, had sat on the floor for more than six weeks after the murder before being secured and processed; photographs show that it had been moved between the murder and its eventual collection. The clasp was the only DNA evidence placing Sollecito at the scene of the crime; no DNA put Knox on the scene at all.

The supposed murder weapon, a long kitchen knife, was found in the home of Sollecito, in his kitchen knife drawer. The knife held little DNA and, according to the experts, the local authorities had not handled the tests properly to compensate.

The tiny bit of DNA on the knife produced results so spotty that they would be legally barred from being introduced in a US court. But not, evidently, an Italian one. Yeesh. Read more at Ars Technica.

Image courtesy of Kuzma / istockphoto

CATEGORIZED UNDER: Health & Medicine
  • Dawes Mangus

    DNA evidence has become irrelevant, moot and insignificant since the day they acquited OJ Simpson. Apparently, odds like 1 in 500,000,000 or something like that is “reasonable” doubt.

  • bmull

    1. No DNA evidence was thrown out. The independent experts deemed the DNA on the bra clasp and knife “unreliable”. Reliability should depend on context. For example a contaminated sample should not be used to establish paternity. But a bra clasp with the DNA of someone who DENIED EVER BEING IN THE ROOM is highly suspicious. Similarly a knife with the DNA of the victim IN A SUSPECT’S APARTMENT is suspicious even though the amount of DNA was below that normally considered reliable.

    2. DNA samples do not have to be taken immediately. A Woolly Mammoth may be cloned from DNA deposited 10,000 years ago. A sample that has been touched by multiple people can still be used, although as the number of contributors increases the chance of a false positive increases. For example, while a clean sample has statistical power of << 1 in a billion, it was "only" possible to identify Sollecito's DNA with a likelihood of <1 in 10,000.

    3. Even if the double DNA knife were thrown out for being too marginal, there is still loads of other evidence–DNA and otherwise–against all three defendants. The knife was just as problematic for the prosecution as it was for the defense. It didn't have blood on it for one thing. For another, why would the killers bring it back to Sollecito's apartment and not the other knife? The truth is probably that the knife was not used in the murder, but came into contact with Meredith's DNA during the destruction of evidence at Sollecito's apartment.

  • anastasia

    I do not see how Sollecito’s DNA could somehow find its way into Meredith’s room where he has never been before and thereafter find its way to the bra clasp of the victim. The jurors on the original case considered the argument about tainting and rejected it. The experts on appeal had never had on-hand forensic testing experience. They were academicians. Further, the arguments raised on appeal were essentially the same as the arguments raised by the defendant’s experts at the original trial, which were considered and rejected by the original trial court with complete and full explanation and analysis. . Lastly, the experts on appeal did not wish to retest, and the court denied retesting. Therefore, their testimony consisted of yesterday’s mashed potatoes, and the only thing new were a set of 12 new juror ears. I certainly can understand their resistence to retesting? What if the appeals experts made the same findings? What if they botched it up because of their lack of experience? Re-testing would be too unpredictable for a decision that was very clearly made before the appeal was heard. . Far safer to testify about deviations from standards of care, deviations that truly were not shown to have any real significance to the case at hand.

  • worried

    I don’t know whether Sollecito & or Knox are guilty or innocent, There certainly seems
    to be doubt, if not outright exoneration.

    The prosecutor in a bizarre unrelated story, had been involved in a long running case
    of a serial killer who was never caught, he killed couples in Lovers Lanes over a 10-20 year
    period, and some journalists, one American the other Italian, starting writing a book
    how the prosecutors and police bungled the case, the next thing you know, the journalists
    were interrogated and arrested, and threatened with being charged as the serial killer,
    just because they were writing a book on the case, and had been unflattering in their portrait of the investigation. Any prosecutor who would do that is suspect in my view.
    Some of the responses I have read remind me of the so called, law and order views of
    the American south, where so many innocent people have been released from death row
    after it was found they were innocent with DNA tests from Project Innocence and Barry Scheck.

    These two kids may have committed the murders or they may not have. I don’t know
    but people who feel they always know who is guilty, are probably a lot scarier than
    any teenage killer. We all want to be safe from violent crime, but it appears a large minority of perhaps 5-10 percent of all the people in the USA convicted of murder are
    actually innocent. That is a lot of mistakes. Mind you that is with the higher safeguards
    of U.S. laws and DNA testing. In other words, even with safeguards to protect the innocent
    in America, thousands of people convicted of murder, rape and robbery are actually innocent. So I conclude one of two things, Policemen and prosecutors are not always
    honorable in search of the truth, or B, as a group they aren’t that bright. Take your

    I am shocked people would think the evidence against Solliceto and Knox was sufficent to
    convict them. Sure it’s possible, but it wasn’t proven, not to any cautious thoughtful person

  • Jason

    Reply to bmull –
    The independent experts said not only that the bra clasp was likely to be contaminated, but also that the result of the test had been wrongly interpreted. They concluded that there was no reason to believe that Sollecito’s DNA was ever on the clasp. Furthermore, the DNA profiles of around a dozen unidendified people were also on this clasp (which as everyone knows had been kicked around the apartment for six weeks). The police did not look for them. They were not interested in establishing the facts but only in framing Sollecito.
    As you are forced to admit, the supposed Kercher DNA on the kitchen knife is similarly discredited. The theory that this knife was the murder weapon was full of holes from the start.
    There was not “loads of other evidence”. What there was was loads of cooked-up, bogus evidence, similar to the knife and the bra-clasp. For example mixed-DNA samples were presented as evidence of mixed blood (they are not). Positive luminol reactions to footprints were presented as evidence of blood (they are not).
    All this supposed evidence was only thrown together AFTER Knox and Sollecito had been arrested, in order to justify their arrest. Presumably this is because the police were too embarassed to admit that they kept arresting the wrong people, while they had failed to arrest Guede for repeated burglaries before the murder. The pattern of disinformation strongly suggests deliberate deception by the prosecution, and highlights the complete absence of any real evidence against Knox and Sollecito.
    Reply to anastasia – The fact that the orginal judge and jury accepted the bogus forensic evidence simply underscores their partiality and incompetence. One cannot still cite the conclusions of that trial as valid, when those conclusions have now been decisively overturned. In that trial, the defence requested an independent review of the forensic evidence, similar to the one permitted in the appeals trial, but this request was rejected by the judge. Instead of “complete and full explanation and analysis”, the trial simply accepted the arguments presented by the prosecution.
    The reason why there was no retesting of the clasp and knife is that the evidence had been destroyed. In the case of the bra clasp, this was because of inappropriate storage – one of the dozens of errors by the police identified by the independent experts.
    Those who continue to insist that Knox and Sollecito are guilty cannot explain why they left no evidence of their presence at the scene or on Kercher’s body. Not just DNA, but no hairs, fibre samples, fingerprints, or anything at all. Nor did they pick up any evidence from the scene on their clothes or bodies. This is simply not credible. Oh, no credible witnesses and no motive as well. The case should never have gone to trial.

  • MikeJ

    Let’s use Occam’s Razor. After the murder the police find the DNA of Rudy Guede, a local small-time criminal and drug dealer, on and in Ms. Kercher and all over her room. Additionally, they find his bloody fingerprints — Kercher’s blood. They do not find any DNA from Knox or Sollecito in the room. Zero. Guede flees to Germany immediately after the murder. The Italian police arrange to have an associate of Guede lure him back to Italy in a phone call, which they tapped. In the phone conversation Guede tells the associate that Knox and Sollecito were not at the apartment when Kercher was killed. Guede comes back to Italy and is tried and convicted of the rape and murder and gets over 30 years. Where do you need Knox and Sollecito and sadistic sex games in all that? Also remember that Guede did not mention the two as participants in the murder until he was offered a deal to cut his sentence in half if he would testify that they had been involved.


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