Prosecutors at Amanda Knox's appeals trial battled it out Saturday with independent forensic experts who say some of the key DNA evidence used to convict the American student of murdering her British roommate was unreliable and possibly contaminated.

Prosecutor Manuela Comodi sought to undermine the expert's conclusions and show that the forensic evidence used to convict Knox could stand. The experts -- who were appointed by the court to review the evidence and the procedures used to obtain it -- maintain that the original investigation was marked by some glaring errors. They have mentioned more than 50, including the wearing of dirty gloves in collecting evidence.

Knox was convicted of sexually assaulting and murdering Meredith Kercher in 2007 the apartment the two shared in Perugia and sentenced to 26 years in prison. Knox's co-defendant and ex-boyfriend, Raffaele Sollecito of Italy, was convicted of the same charges and sentenced to 25 years.

Knox, 24, and Sollecito, 27, have denied wrongdoing and have appealed.

It was the trial's last hearing before the summer break, and much of the debate Saturday centered on a kitchen knife the prosecutors believe to be the murder weapon.

In the first trial, prosecutors maintained that Knox's DNA was found on the knife's handle and Kercher's DNA was found on the blade. They also say Sollecito's DNA was found on the clasp of Kercher's bra, mixed with the victim's.

But the independent experts told the appeals court earlier in the week that the collection of evidence fell below international standards. They said the knife was not properly sealed or kept after it was found at Sollecito's house, opening the way to possible contamination.

The experts said that the DNA on the blade could not be attributed with certainty to Kercher. They reviewed the procedures used to test the original DNA material, concluding that the genetic quantity was below the minimum amount necessary for the test to be considered reliable.

"There is a complete genetic profile, but it's not reliable," testified Carla Vecchiotti, one of the court-appointed experts. "We don't know if Meredith's DNA was on it or not."

Comodi insisted that the genetic profile found on the blade should not be tossed out. She argued that no amount of contamination could have led to Kercher's DNA on the blade.

"It's not that easy to leave DNA in places," Comodi said.

Fanning herself in the hot courtroom over five hours of relentless questioning, the prosecutor challenged the experts over what is considered the minimum quantity of DNA required for a test to be carried out, suggesting that various scholars differ on the threshold.

The experts have also suggested a risk of contamination on the bra clasp, which was found at the crime scene 46 days after the murder. They have shown pictures of a dirty glove handling it.

The experts, however, admitted Saturday that they could not rule out that Sollecito's DNA was part of the mixed DNA found on the clasp. But they insisted it may well have been the result of contamination.

"Today, even though the prosecutors tried to make confusion, I don't think it worked," said Knox's mother, Edda Mellas. "The experts are sticking by their report. There is no DNA that connects Amanda and Raffaele to the crime."

The other parties had a different take on the hearing. Francesco Maresca, a lawyer representing the Kercher family, said the cross-examination showed the experts aren't infallible.

"At the end of the day, this review doesn't have solid foundation," he claimed. "It's the experts' very, very personal opinion."

The appeals court on Saturday allowed the police chief who conducted the original investigation, Patrizia Stefanoni, to take the stand in future hearings, granting a prosecutors' request. Forensic consultants used by the parties, who followed the DNA reviewed, will also be heard.

The trial will resume Sept. 5, when Maresca will cross-examine the experts. Vecchiotti and her partner in the review, Stefano Conti, are forensic experts from La Sapienza university in Rome.

The experts' strong criticism of the investigation methods led to a letter of protest by forensic police, which was read in court by the presiding judge. Piero Angeloni, head of the Italian police forensic unit, rejected the accusations, which he said hurt the image of police and undermined their work.