Italian police found DNA traces of murdered British student Meredith Kercher and her American flatmate Amanda Knox on a knife belonging to Knox's boyfriend, an investigative source said on Thursday.

Kercher, a 21-year-old exchange student, was found dead in her bedroom with a deep cut to the throat in the university city of Perugia, about 130 km (80 miles) north of Rome, on Nov. 2.

Knox, 20, Knox's Italian boyfriend, Raffaele Sollecito, 24, and a Congolese man, Lumumba Diya, 37, who runs a bar in Perugia, have been in police custody since Nov. 6.

The investigative source told Reuters the knife concerned was a kitchen knife that belonged to Sollecito.

However, Sollecito's lawyers said the DNA found on the knife came from "generic biological material" and there was no trace of Kercher's blood on it.

Sollecito's own DNA was not found on the knife which investigators took from his house and was not the one he was carrying at the time of the crime, they said in a statement.

Police suspect a sexual motive and Italian and British newspapers have speculated that Kercher, on a year's study trip from Leeds University, was killed because she refused to have sex with one or more assailants.

The 21-year-old's body was flown back to Britain on Sunday.

The three suspects have not been formally charged over the killing. All say they are innocent.