Amanda Knox, the American student accused of the murder of Meredith Kercher, her British flatmate, was not only present at the "group crime" but opened the door to let in an African immigrant who has been already sentenced for his part in the killing, an Italian judge has said.

The trial of Ms Knox, 21, and Raffaele Sollecito, 24, her Italian former boyfriend, on charges of murder, sexual assault and theft opened two weeks ago in Perugia and resumes in ten days time. Both deny the charges.

Ms Knox claims that she spent the evening and night of the killing in November 2007 at Mr Sollecito's flat smoking cannabis with him and watching films on the internet. Both say they only went to the whitewashed hillside cottage Ms Knox shared with Ms Kercher near Perugia's University for Foreigners the following day, when Ms Kercher's body was discovered by police.

Judge Paolo Micheli, releasing a report on his reasons for sentencing Rudy Guede, 22, to 30 years in prison in October for his part in the murder, said the killing was "a group crime". Guede had not himself cut Ms Kercher's throat. But there was "cast iron proof" that he had taken part in the murder, even if he did not strike the "mortal blow".

Under Italian law a judge has to outline the "motivation" behind his verdict. Unlike Ms Knox and Mr Sollecito, Guede, who is from the Ivory Coast but was brought up in Perugia and turned to petty crime, opted for a "fast track" trial in the hope of a reduced sentence.

Judge Micheli was also the pre-trial judge who in October said there was enough evidence against Ms Knox and Mr Sollecito for them to be sent for trial. The prosecution alleges that Guede tried to sexually assault Ms Kercher while Mr Sollecito held her down and Ms Knox toyed with a knife against her throat, which she then used to stab her. Judge Micheli said he accepted that there was "complicity" between the assailants, but said some aspects of the prosecution reconstruction were "fantasy".

Reconstructing the crime in his 106-page report, Judge Micheli said the first blow was struck at Ms Kercher while she was standing up. He said she was killed because she refused to take part in a sexual game which "escalated into violence and got out of control".

Judge Micheli said Guede was "a liar", and there were "no extenuating circumstances". "Even someone who wanted to believe him would find it impossible," the judge wrote. He added: "It is credible that Guede entered the house because he was let into it by someone else, and that someone could only be Amanda Knox."

He said there had been an "agreed plan" to satisfy "sexual instincts" which ended in "murderous intent". Guede had continued to try to assault Ms Kercher sexually even when a knife was produced and even when the knife "sank deeper into her neck" the judge said. Guede had not completed the sexual act only because of Ms Kercher's "screams of pain and fear".

The prosecution in the trial of Ms Knox and Mr Sollecito alleges that Ms Knox arranged for Guede, who had made clear that he was attracted to Ms Kercher and wanted to have sex with her, to come to the cottage when she knew her flatmate was there.

Judge Micheli said the statements Guede, who fled to Germany after the murder, had made following his arrest and extradition to Italy were "nothing more than a colossal accumulation of contradictions and attempts to throw investigators off the track".

In his haste to flee Guede had bumped into a couple near the cottage who had testified to police, the judge said. Ms Knox and Mr Sollecito meanwhile had been seen at a square above the cottage by a homeless man, and apparently waited there "to see if police arrived", the judge said.

He said that because of "complicity" between the three, Guede had "never once mentioned the name Amanda" until late into the inquiry, when he said he had heard Ms Knox's voice at the door and seen a man "resembling" Mr Sollecito. Guede is due to give evidence later in the trial of Ms Knox and Mr Sollecito, which is expected to last at least until the autumn.

Judge Micheli said Guede had had "no intention of saving" Ms Kercher's life as she lay bleeding to death. He noted that neighbours had testified that they clearly heard a woman screaming in agony inside the cottage late at night. In his defence Guede had claimed that he was in the bathroom with stomach pains when Ms Kercher was murdered. The judge said this was untrue.

Ms Kercher, a Leeds University student from Coulsden in Surrey, was in Perugia as part of a year-long Erasmus European exchange programme. She had only been in Italy for two months when she was murdered.

Times Recommends

• Both sides blame US for worsening relations
• Stieg Larsson: losing the plot over his cash
• Odd creature paralysed by incompatible parts