And the victim's DNA is on the 6 1/2-inch blade, CNN reported Thursday a "source close to the prosecution" as saying.
These "unmistakable facts" show the knife played a role in the slaying of Meredith Kercher, said the source, who CNN said "did not wish to be identified discussing an ongoing case."
Kercher, 21, of Surrey, England, was found partially clothed, with her throat slit Nov. 2, 2007, covered in a duvet in the house the two students shared with two other women in the college town of Perugia, Italy.
Amanda Knox, 21, a linguistics major from the University of Washington in her junior year abroad, is accused of killing Kercher with her boyfriend at the time, Rafaelle Sollecito, 25, and a third man in a drug-fueled sex game that went awry.
She and Sollecito, who also is on trial, maintain their innocence. The third man, Rudy Guede, has pleaded guilty.
Knox's lawyers accuse investigators of shoddy police work and tampering with the crime scene.
Anne Bremner, a lawyer and former prosecutor working with the group Friends of Amanda, told CNN the knife could not be the murder weapon.
"It doesn't match the size or shape (of the wounds)," she said.
Closing arguments in the case, which went to trial in January, begin Nov. 20. The jury is expected to begin deliberating Dec. 4.