ON THE eve of the verdict of the high-profile trial in Perugia, Italy, in which Amanda Knox and Raffaele Sollecito are charged with the murder of British student Meredith Kercher, DNA evidence brought by the prosecution is being questioned by nine US specialists in DNA forensics.
The trial is in its final stages, with a verdict expected on 7 December. Knox and Sollecito are pleading not guilty. In an open letter, the US specialists outline their concerns with two pieces of DNA evidence that are central to the case against Knox and Sollecito. The defence has already suggested that the amount of DNA allegedly linking the pair to the crime is too small to be definitive. The letter raises the possibility that this DNA was introduced through contamination of the evidence and concludes that the DNA test results 鈥渃ould have been obtained even if no crime had occurred鈥�.
Two years ago, Kercher was found dead, with cuts to her throat, in the accommodation she shared with Knox in Perugia. Although a third suspect, Rudy Guede, has been convicted of her murder, the prosecution claims that Knox and Sollecito helped. The case against them largely hinges on two pieces of DNA evidence. One is from a knife recovered from the kitchen drawer at Sollecito鈥檚 apartment. The other comes from a portion of a clasp that was cut away from the bra Kercher was wearing. The forensic specialists question both.
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In respect of the knife, the letter says contamination from other DNA present in the lab that did the analysis cannot be ruled out. The initial method used was standard: the DNA was amplified, then analysed using electrophoresis. This generates a graph consisting of a series of peaks, whose heights represent how much of certain DNA snippets are present. Taken together the peaks create a DNA 鈥渇ingerprint鈥� unique to an individual.
The lab says that DNA taken from the knife鈥檚 blade produced a series of peaks that matched Kercher鈥檚 DNA, while DNA from the handle produced peaks that matched Knox鈥檚.
To minimise the risk that some peaks arise from contamination, most US labs only count peaks above a height threshold of 150 relative fluorescence units (RFUs) and all dismiss those below 50. The trouble with the DNA found on the knife is that 鈥渕ost of the peaks are below 50鈥�, says Greg Hampikian of Boise State University in Idaho, who signed the letter and reviewed the DNA evidence.
When this happens, samples can be rerun, but this doesn鈥檛 appear to have been done in the Knox and Sollecito case. This means contamination cannot be ruled out, the open letter claims. The same lab may also have been running DNA profiles from other evidence in the case at the same time, it says, and tiny amounts of this could have contaminated the knife samples.
What鈥檚 more, a sensitive chemical test for blood on the knife was negative, and it is unlikely that all chemically detectable traces of blood could be removed from the knife while retaining sufficient cells to produce a DNA profile. 鈥淣o credible scientific evidence has been presented to associate this kitchen knife with the murder of Meredith Kercher,鈥� the letter concludes.
Evidence from the clasp is equally inconclusive, according to the letter. What looks like a mixture of different people鈥檚 DNA was found on it, and Sollecito could not be excluded. However, because Sollecito had visited the women鈥檚 home several times before the murder, his DNA could have made its way onto the clasp 鈥渢hrough several innocent means鈥�, the letter says. Neither Sollecito鈥檚 nor Knox鈥檚 DNA was found on the remainder of the bra, other items of Kercher鈥檚 clothing, objects collected from Kercher鈥檚 room, or in samples from her body. Guede鈥檚 DNA was found everywhere, the letter points out.