An Italian court rejected a prosecutor's request Wednesday for new testing of crucial DNA evidence in the appeals trial of Amanda Knox -- a positive development for the American student who is seeking to overturn her murder conviction.

The decision to deny more DNA testing was a blow to the prosecution, which had sought to counter the results of a court-ordered independent review that harshly criticized how DNA evidence was used in the case.

Without a clear motive or convincing witnesses, the DNA evidence is crucial, and much of the appeals outcome hinges on it.

The court presided over by Judge Claudio Pratillo Hellmann said closing arguments to begin on Sept. 23, with the prosecution going first, followed by civil plaintiffs and the defense.

Knox was convicted in December 2009 of sexually assaulting and murdering her British roommate Meredith Kercher while they were studying in Perugia and sentenced to 26 years; Raffaele Sollecito, an Italian who was Knox's boyfriend at the time, was convicted of the same charges and sentenced to 25 years.

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Both deny wrongdoing and are appealing the lower court verdict.

In the first trial, prosecutors maintained that Knox's DNA was found on the handle of a kitchen knife believed to be the murder weapon, and that Kercher's DNA was found on the blade. They said Sollecito's DNA was on the clasp of Kercher's bra as part of a mixed trace that also included the victim's genetic profile.

The appeals court ordered an independent review, which found that much of that evidence was unreliable and possibly contaminated. The review was at the center of several fiercely debated hearings in the Perugia courtroom, with police defending their investigation.

Pratillo Hellmann said the discussion had been thorough enough for the court to form an opinion. New testing would be "superfluous," he said, rejecting the request made earlier in the day by Prosecutor Manuela Comodi.

The court also rejected another prosecution request to put back on the stand a witness who had previously testified that his brother, a fugitive, had killed Kercher during a botched burglary. The witness, a jailed Naples mobster called Luciano Aviello, announced he wanted to retract and was questioned by Comodi in prison in July.

The court ruled that transcriptions of that questioning would suffice.