PERUGIA, Italy – DNA evidence at the center of a murder trial in Italy of an American student accused of killing her roommate is unreliable, according to a forensic expert testifying for the defense Monday.

Adriano Tagliabracci took the stand in the trial of Amanda Knox and co-defendant Raffaele Sollecito, which resumed after a summer break, in Perugia, central Italy.

Knox and Sollecito are charged with murder and sexual violence in the 2007 slaying of Meredith Kercher, from Britain, who was stabbed in the throat in her bedroom in the house she shared with Knox. The defendants deny wrongdoing.

Tagliabracci, who was called by Sollecito’s defense, contended that DNA traces allegedly belonging to Kercher found on a knife that might have been used in the slaying were “too low” to be attributed with certainty. The knife was analyzed by forensic police.

According to prosecutors, Knox’s DNA was found on the knife’s handle, while Kercher’s DNA was found on the blade. The knife was found at Sollecito’s apartment.

“It’s a disputable element and it would have been better to stop the test. The traces are so low that any result would be open for discussion,” Tagliabracci said.

Prosecutors also say they found Sollecito’s DNA on the clasp of Kercher’s bra, although his defense team contend that the evidence might have been inadvertently contaminated during the investigation – an assertion Tagliabracci also made during his testimony.

Earlier Monday, the court rejected defense lawyers’ requests to throw out Knox’s and Sollecito’s murder indictments that are partly based on DNA evidence.

Defense lawyers say some documentation supporting the attribution of DNA samples were not made available to the defense promptly.

Italian prosecutors say forensic and DNA experts have followed correct procedures while submitting the results of DNA tests to the court.

Proceedings were adjourned to Friday, when the knife that might have been used in the slaying will be shown in court. Prosecutors allege that Kercher was killed during a sex game and that Knox fatally stabbed her in the throat.