Three people are being questioned about the death of Briton Meredith Kercher who police suspect was murdered after a violent sexual attack.
Meredith Kercher may have met her killer at a party, police believe
According to reports, all three had been at the house in Perugia, Italy, where Miss Kercher, 21, of Coulsdon, south London, was found dead.
They include her American housemate, Amanda Knox, aged 20, and Miss Knox's boyfriend, Raffaele Sollecito, 24.
Also questioned was Patrick Lumumba, 37, a Congolese immigrant.
Arturo De Felice, Perugia's police chief, told a press conference that she died fighting off a sexual attack.
He said that Miss Kercher was a "victim and nothing more".
Asked by reporters if one of the three people being questioned had confessed to the crime, the police said this was a "valid hypothesis".
The BBC's Christian Fraser said that detectives investigating the case have always believed that those closest to Miss Kercher knew more about the case than they were prepared to reveal.
He added that Miss Kercher's friend and fellow student, Sophie Purton, had been refused permission to leave Italy.
Amanda Knox was Meredith Kercher's flatmate
Miss Kercher's body was found on Friday in the bedroom of her apartment.
She was partially-clothed and under a duvet and is thought to have been stabbed with a pocket knife.
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.
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 Kercher, who had been on a study year from Leeds University 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.
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.
He said: "Meredith was just not that sort of person.
"She would never have got involved in anything like that.
"I think I am dealing with it and then I see cards and pictures or I hear her name mentioned and it sets me off crying again."
Friends of Miss Kercher held a candlelit vigil in her memory at Perugia's cathedral on Monday night.
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In a statement, her family said they were "devastated" at losing "a loving daughter, sister and a loyal friend".
It said: "She was pursuing her dream and we can take some comfort in knowing that she has left us at what was a very happy time in her life.
"We feel it is no exaggeration to say that Meredith touched the lives of everyone she met with her infectious, upbeat personality, smile and sense of humour.
"The sheer volume of tributes that have poured in during the past few days bear testament to the special girl we will all remember."
The statement also appealed for anyone with information to come forward "to help us bring to justice the person who has destroyed so many lives".