Amanda Knox legal fight highlights fallibility of DNA forensics

In the wake of the Amanda Knox case, a DNA expert for the defence says greater scrutiny of crime labs is needed to avoid further injustice

GALILEO famously declared that “science proceeds more by what it has learned to ignore than by what it takes into account”. As DNA consultant for the defence in the Amanda Knox case, I was constantly reminded of the pertinence of this observation during her legal battle in Italy.

Knox, along with Raffaele Sollecito, was definitively cleared of killing Meredith Kercher earlier this year, but only after a long fight that had at its heart the ability of forensic science and the judiciary to know what to ignore.

On the day of the murder in 2007, police collected many samples from the room where Knox’s housemate Kercher died. Knox and then boyfriend Sollecito were held on the basis of the prosecutor’s gut instinct, but when fingerprints and DNA from the scene were analysed, only two profiles were identified: those of the victim and Rudy Guede, a man known to police. He was convicted of murder, but the prosecutor still pursued Knox and Sollecito.

One piece of evidence emerged as crucial: a kitchen knife at Sollecito’s house. It didn’t match many wounds on the body and tested negative for blood. DNA from Knox was on the handle – she had cooked with it. But on one swab from the blade, a minuscule trace of DNA was detected, just once during many analyses. It had some that was consistent with the victim’s. This finding was never repeated, despite many attempts. The debate was about whether or not that single result was reliable.

For any scientific procedure, it is crucial to know how often it gets things wrong as well as right. In Knox’s case, the DNA on the blade came from so few molecules that analytical instruments were pushed to read below the level that the FBI, my lab, or anyone I knew would go. We asked the Italian lab to supply validation of such a sensitive measurement, but they never complied. Despite this, Knox was convicted. DNA experts in the US spoke out and a new study on the knife was then ordered in Italy. This failed to repeat the DNA finding, and Knox and Sollecito were freed on appeal in 2011. Then in 2014, the conviction was inexplicably reinstated. The final hope rested with the supreme court this March. Justice would require it to see that there was no credible DNA evidence. Apparently it did.

Knox and Sollecito waited years to be properly cleared. Calls followed for global standards on use of low copy number DNA. But we also need better ways to weigh up new forensic techniques and issue warnings if required.

My research has shown that DNA tests are prone to subjectivity in labs. So forensic facilities must put out validation records and error reports, and open data up to scrutiny – anything less creates too high a risk of false convictions.

Original report here


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