The ANSA news agency said the finding was contained in Judge Paolo Micheli's written explanation of his October ruling to convict one of the three suspects in the slaying of Meredith Kercher. In Italy, judges issue explanations of their decisions months after they are handed down.
It was not immediately possible for The Associated Press to obtain a copy of the ruling after business hours Monday.
Micheli sentenced Ivory Coast national Rudy Hermann Guede to 30 years in prison after he underwent a fast-track trial at his request.
Two other people, American Amanda Knox and Italian Raffaele Sollecito, are on trial on the same charges of murder and sexual violence in Kercher's 2007 death. All three have denied any wrongdoing.
Kercher was found stabbed to death Nov. 2, 2007 in the apartment she shared with Knox in Perugia, 115 miles north of Rome where both were exchange students.
In the decision, Micheli said Kercher was the victim of "an agreed-upon plan to satisfy sexual instincts" that got out of control, ANSA reported. Micheli said Guede participated "actively" in the assault but that there were others involved as well.
The ruling echoed the contention of prosecutors, who have said Kercher was killed in what began as a sex game.
Prosecutors allege that Sollecito held Kercher her by the shoulders from behind while Knox touched her with the point of a knife. They say Guede tried to sexually assault Kercher, and then Knox fatally stabbed her in the throat.
At the start of Knox and Sollecito's trial earlier this month, Sollecito's attorneys argued that justice had already been served with Guede's conviction. Attorneys for Knox and Sollecito say their clients were not in the apartment at the time of Kercher's slaying.
The next hearing in the case is scheduled for Feb. 6.