The American flatmate of murdered Meredith Kercher has claimed she covered her ears as the British student screamed, Italian police have said.
Meredith Kercher was on an exchange study programme
Amanda Knox, 20, has given a partial confession, but keeps changing her story, police in Perugia say.
Miss Knox, her boyfriend Raffaele Sollecito, 24, and Congolese immigrant Patrick Lumumba, 37 are all being questioned about the murder.
Miss Kercher, 21, from Coulsdon, south London, was found with her throat cut.
Perugia's police chief Arturo De Felice said all three suspects had given different versions of events.
Miss Knox, he said, "crumbled" under interrogation and claimed she was in the kitchen of the apartment she shared with Miss Kercher when Mr Lumumba went into the Briton's bedroom alone.
She told police she then heard screaming from the room, but covered her ears to block out the noise.
The BBC's Christian Fraser, in Perugia, said Miss Knox had told police: "I can't remember how long they were in there, but at one point I heard Meredith scream.
"I was horrified. After that I don't remember anything. I was very confused."
Mr De Felice said all three suspects were present at the murder scene and all played a different role.
Their mobile phone records were vital to the investigation, he said, telling our correspondent: "If I told you I was in Rome, but the record showed I was in Paris, then you might begin to ask questions."
The police chief said all three suspects would go before a judge on Thursday to be formally arrested.
They are being held separately and have not had access to their lawyers.
Earlier reports said Miss Knox and Mr Sollecito were at the house when police found Miss Kercher's body in her bedroom on Friday.
She was partially-clothed and under a duvet and, according to the coroner, her throat was cut with a pocket knife or a sharp piece of glass.
Mr De Felice told a press conference she was murdered fighting off a violent sexual attack.
He said she was a "victim and nothing more".
Luca Lalli, the medical examiner, said there was evidence that she had taken part in sexual activity before her death, but did not confirm reports that she had been raped.
Perugia is home to many students from overseas
A spokesman for Leeds University where Miss Kercher was a student said: "The whole [Italian] department is in shock that such a terrible thing should have happened.
"We will remember Meredith as a beautiful, clever and happy young woman, who was serious about her studies and popular with her peers."
Staff and students at the university will hold a vigil in memory of Miss Kercher on Wednesday evening.
Detectives have five days to question the three people being held in connection with Miss Kercher's death.
Mr Lumumba had been in the country since 1988, while Mr Sollecito was a student at the local university. Miss Knox is from Seattle, Washington.
Miss Kercher, who had been on a study year in Italy as part of the Erasmus programme, is known to have gone to at least one party on Halloween last Wednesday, the night before she died.
The next evening, she had dinner with a female friend, and went home in the middle of the evening.
Officers have been carrying out checks for fingerprints on two mobile phones belonging to Miss Kercher, which were found at or near the scene.
An elderly woman found one mobile abandoned in her garden and handed it to police.
Detectives traced it to the flat, where they kicked in the door and discovered Miss Kercher's body.
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They are also studying a footprint left in the blood in her bedroom.
The Daily Telegraph quoted her father, John Kercher, as saying that he had found it "very difficult" to learn of the circumstances surrounding her death.
Earlier reports suggested she may have died after a sexual encounter went wrong.
But Mr Kercher said: "She would never have got involved in anything like that."
Friends of Miss Kercher held a candlelit vigil in her memory at Perugia's cathedral on Monday night.
Her parents later visited the cathedral where Mr Kercher left a tribute to his daughter which read: "I love you forever Meredith. All my love, Dad."