Amanda Knox delivered the fatal knife blow to UK student Meredith Kercher, the Italian judges who reinstated the guilty verdict against her have said.
Explaining their ruling in a 337-page document, the judges said Knox and her Italian ex-boyfriend, Raffaele Sollecito, killed Miss Kercher after a violent argument.
It adds that the British student's wounds indicate multiple aggressors and that the two exchange students fought over money on the night of her death.
Knox and Sollecito were originally convicted of the murder in 2007 but were acquitted by the High Court in 2011 and released from prison. The appeal court convicted them for a second time in January.
The document said: "It is a matter of fact that at a certain point in the evening events accelerated; the English girl was attacked by Amanda Marie Knox, by Raffaele Sollecito, who was backing up his girlfriend, and by Rudy Hermann Guede, and constrained within her own room."
Knox has insisted there is "no logic" to an Italian court's decision to reinstate her conviction for the murder of British student Meredith Kercher.
In a written statement Knox said "I am innocent."
She said forensic evidence refutes the appeal court's theory that more than one person attacked Kercher and that a small kitchen knife was used in the murder, the Associated Press said.
Knox says she will now take the case to Italy's Supreme Court for an appeal.
If the court confirms the conviction a long extradition fight is expected for Knox, who is now living back in America.
Ms Kercher, 21, was found dead in the flat she and Knox shared in the town of Perugia. Her throat had been slashed and she had been sexually assaulted.
In its explanation the appeal court said that a third person convicted of the murder, Rudy Hermann Guede, did not act alone, and cited the nature of the victim's wounds, as well as finger imprints on her body indicating she had been restrained.
The said it had evidence of a fallout between the two roommates, including statements by Guede under police questioning that Ms Kercher had accused Knox of taking money from her room.
It ruled out a sex game gone wrong because it was not in Ms Kercher's character.
Guede was convicted in a separate trial of sexually assaulting and stabbing Kercher. His 16-year sentence - reduced on appeal from 30 years - was upheld in 2010 by Italy's highest court, which said he had not acted alone.