Meredith Kercher was killed by more than one person in what may have been a group sex game that spiralled out of control, Italy's highest court has ruled.
The Supreme Court in Rome said "clues" that suggested Amanda Knox and her Italian boyfriend might have had something to do with the murder were "underestimated" by the appeal court that quashed their convictions.
Judges said there was a good possibility that the one person who is in prison for the murder, Rudy Guede, did not act alone. He "was not the only culprit", the Supreme Court said.
It released a 74-page document in which it explained its decision, in March, to overturn the acquittals of Knox and Raffaele Sollecito, her ex-boyfriend, and order a retrial.
It is standard for Italian courts to release such explanations two to three months after their initial ruling.
Kercher's murder, in Perugia in 2007, might have been the result of "an erotic group game that spun completely out of control", the court said.
The retrial will be held in Florence, probably starting early next year.
Knox, back studying at Washington University in Seattle, is not expected to return to Italy to attend.
The judges said she should be held to account for wrongly accusing an innocent man of the murder.
Patrick Lumumba, a Congolese barman who lives and works in Perugia, spent time in jail before being released after producing a credible alibi for the night of the murder.
"Even though she was young, Miss Knox was a mature girl, born and raised in a country where one is not allowed to wrongly accuse another person to free oneself from a difficult situation," the judges wrote.
The Supreme Court judges lent support to the prosecution, which insisted in the original trial that Knox, Sollecito, and Ivory Coast-born Guede, a local drifter and small-time drug dealer, submitted Kercher to a violent sex game, eventually killing her by stabbing her in the throat.
Kercher, from Surrey, who was studying at Perugia's University for Foreigners for a year, was found dead in the house she shared with Knox and two Italian women. Knox and Sollecito were found guilty of her sexual assault and murder at a trial in 2009 but later acquitted on appeal.
Those appeals were in turn overturned by the Supreme Court, which ordered the retrial. It said there had been "shortcomings and contradictions" in the lower appeal court's decision to acquit the former lovers.
Guede was found guilty of sexually assaulting and murdering Kercher in a separate trial and condemned to 30 years in prison - a sentence later reduced to 16 years on appeal.
Knox and Sollecito continue to insist they had nothing to do with the killing. "If there was an erotic game, most certainly Raffaele Sollecito and Amanda Knox were not part of it," Sollecito's lawyer, Giulia Bongiorno said. Her client had nothing to fear from a retrial because he was innocent. The new trial would "only bring out the strangeness of ... the allegations."