The American flatmate of British exchange student Meredith Kercher allegedly confessed to police that she heard her die.
Amanda Knox, 20, reportedly told officers she covered her ears to the sound of screaming coming from the student's bedroom in the house they shared in Perugia, Italy.
In her alleged confession, published in the Corriere Della Sera newspaper, she claimed Miss Kercher was in her room with suspect Lumumba "Patrick" Diya when she started to scream.
She said: "Patrick and Meredith were in Meredith's room, while I had stayed in the kitchen. I can't remember how long they were together in the room but I can only say that at a certain point I had heard Meredith screaming and I was scared so I covered my ears.
"After that I don't remember everything, my head is very confused. I don't remember if Meredith called out or if I heard thuds because I was upset, but I can imagine what was happening."
Italian police believe Miss Kercher, 21, was killed after she refused to have violent sex with her friends.
Knox, from Seattle, Washington, her Italian boyfriend, student Raffaelle Sollecito, 24, and Diya, 37, originally from the Congo, were all arrested yesterday.
They are being held on suspicion of conspiracy to commit manslaughter and to commit sexual violence.
Knox's mother Edda Mellas was travelling to Perugia to support her daughter after her housemate's horrific murder when she was told she was now a suspect in the case.
Correire Della Sera said Knox "confessed" to officers early yesterday morning, saying: "I want to talk about what has happened because this event has profoundly upset me and I am very scared of Patrick, the African man who owns the pub Le Chic where I work occasionally."
The student claimed she met Diya on November 1 and went back to her house with him.
She allegedly said: "I can't remember whether my friend Meredith was already home or whether she came back later. What I can say is that the two of them went off together."
Knox claimed she spent the night at Sollecito's house but returned the next morning to find the door of her house open, according to the Italian paper.
She also reportedly said she had seen Diya again on Monday morning, when he asked her about the police investigation.
Sollecito's account was also published in the paper, which claimed he had changed his story between Monday and Tuesday night.
Officers were allegedly told how he and Knox, who he had known for two weeks, had been out together the night Miss Kercher died.
He came home around 9pm but his girlfriend stayed out and then came back to his house at around 1am in the morning. The next day she went home for a shower but came back saying she was preoccupied because she had found her house door wide open and traces of blood in the bathroom.
His statement apparently then describes how the couple returned to Miss Kercher's house together and discovered the student's door was closed and locked.
Sollecito allegedly said: "First I looked to see if what Amanda had told me about blood in the bathroom was true and I noticed drops of blood in the sink, but on the tap there was something strange, a sort of mix of blood and water, but the rest of the bathroom was clean... the rest was in order."
He goes on: "At that point Amanda went to the big bathroom and came out shaking and hugged me tightly, telling me that first, when she had had a shower, she had seen that it was clean.
"I asked her what was going on and went out to see if I could manage to climb in through Meredith's window."
He allegedly said he tried to break down her door but failed and that the military police arrived shortly afterwards.
The student reportedly added: "In the following conversation I told them a sack of s*** because Amanda had convinced me of her version of events and I hadn't thought of the inconsistencies."
Miss Kercher's parents John and Arline and sister Stephanie, 24, returned from Perugia to the UK today after an overnight visit. Before they left, they took a walk through the city and Mr Kercher laid a single red rose on the steps of its cathedral.
A note left beside it read: "Love you forever Meredith. All my love, Dad."
Staff at the Italian department where Miss Kercher studied spoke today of their shock at her death, and a vigil is to be held at her university tonight.
A university spokesman said: "We will remember Meredith as a beautiful, clever and happy young woman, who was serious about her studies and popular with her peers."
Miss Kercher arrived in Italy at the end of August as part of a one-year Erasmus programme.
She is thought to have died after being stabbed with a pocket knife. Police are still said to be searching for the murder weapon.