Italy's top criminal court has said its decision to order a retrial of Amanda Knox and her ex-boyfriend in the murder of Meredith Kercher was made because their acquittals contained shortcomings and contradictions.
The Court of Cassation also said the possibility that Briton Miss Kercher was killed in a sex game that had got out of hand needs to be revisited.
Knox and her Italian ex-boyfriend Raffaele Sollecito were initially found guilty of killing the 21-year-old Leeds University student, but both were cleared on appeal in 2011.
In March of this year, however, Italy's top court overturned the acquittals and ordered a retrial . That court has only now issued its written reasoning for doing so.
It picked apart the lower court's judgment freeing Knox, saying it contained "shortcomings, contradictions and inconsistencies" and "openly collides with objective facts of the case".
The high court's 74-page document also said the judges who freed Knox undervalued the fact that the American had initially accused a man of committing the crime who had nothing to do with it.
Miss Kercher's body was found in November 2007 in her bedroom of the house she shared with Knox in Perugia. Her throat had been slashed.
Knox and Sollecito have denied any involvement, saying they were not in the apartment at the time.
A young man from Ivory Coast, Rudy Guede, was convicted of the killing in a separate proceeding and is serving a 16-year sentence.
But Guede is not believed to have acted alone.
The high court judges said the retrial would serve to "demonstrate the presence of the two suspects in the place of the crime".
They said hypotheses that must be considered involve "a group erotic game that blew up and went out of control", and urged the retrial to conduct a full examination of evidence to resolve the ambiguities.
No date has been set for the retrial.
Knox, who left Italy a free woman after her 2011 acquittal, is back in Seattle and is not expected to attend the new trial.
Italian law cannot compel her to return as defendants can be tried in absentia.
She has recently released a book titled Waiting To Be Heard.
Sollecito has spent time in Switzerland trying to start a new life, but it has emerged that his residency permit has been revoked by the Swiss authorities.
In his application, he failed to mention his involvement in a criminal case, Italian news reports said.