Remember that knife has not been proved to be the murder weapon.
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Greg Hampikian, from Boise State University in Idaho, US, is one of them. The forensic expert is critical of the way DNA evidence was handled.
"I could see the problem with the case right away," says Dr Hampikian, who adds that he became interested in the case early on.
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Kercher murder: Timeline
1 November 2007: Ms Kercher is killed at her apartment in Perugia. Police find her a day later.
6 November 2007: Kercher's American housemate Amanda Knox is arrested, along with Sollecito and Congolese national Patrick Diya Lumumba.
20 November 2007: Rudy Guede detained in Germany and extradited to Italy. Mr Lumumba released without charge
28 October 2008: Rudy Guede is sentenced to 16 years. A judge rules Sollecito and Knox will face a murder trial
4 December 2009: Ms Knox and Mr Sollecito found guilty of murder and sexual violence, and jailed for 26 and 25 years
3 October 2011: Amanda Knox and Raffaelle Sollecito are acquitted
26 March 2013: Re-run of appeals ordered. Acquittals overturned
Profile: Amanda Knox
Profile: Raffaele Sollecito
A knife recovered from Sollecito's house was found to have Ms Knox's DNA on the handle and a small amount of DNA on the blade "consistent with the victim".
Dr Hampikian, who founded the Idaho Innocence Project at Boise State University, an organisation that investigates claims of wrongful conviction, says: "That is significant because Miss Kercher had never gone to that house, so what is she doing on the blade of the knife?
"While that may seem on its face to be evidence of a crime, in order to substantiate such a small amount of DNA you look for blood, and I can't emphasise enough how small this was - it was just a few cells."
But there was no evidence of blood or any other body fluids found, the Boise State researcher points out.
"You can't really wash the blood off and leave the DNA in any practical sense. That means that the few cells or molecules might have been from the laboratory after they amplified Miss Kercher's DNA," he explains.
This concern was not his alone. There have been claims that the initial evidence was handled using dirty gloves and that investigators entered the crime scene without protective clothing.