DNA Contamination Has Consequences. Ask Amanda Knox.

(Part of the biometric product marketing expert series)

When Thermo Fisher Scientific announced Amanda Knox as one of its speakers at the HIDS 2025 conference (image from the HIDS 2025 conference page), I wondered why. All of knew of Knox was that she was imprisoned for a murder in Italy that she didn’t commit.

I then found the details.

Prosecution Exhibit #36 was a knife discovered in the kitchen drawer of Raffaele Sollecito’s apartment on November 6, 2007. The police claimed this knife, 31 m long with a 17.5 cm blade, to be the murder weapon. It was the only physical evidence linking Amanda Knox to the murder. The Scientific Police claimed to have found Knox’s DNA on the handle and [murder victim Meredith] Kercher’s DNA on the blade and called the knife the “double DNA knife.”

Because DNA proves all, Knox was sent off to prison.

Only there was one problem.

Later re-evaluation of the knife left little doubt that the DNA found on the knife was the result of contamination.

You see, DNA evidence is examined in a lab. So if someone takes DNA from a knife blade and compares it to DNA taken from Amanda Knox, and if the samples have a high probability of a match, then you can make a determination.

But what if there were a mixture of the DNA, and Knox’s sample was mixed with the knife sample at some point, or misidentified? Then Knox’s DNA would match to Knox’s DNA, but that may have nothing to do with the DNA that was originally on the knife.

And you know nothing.