The flatmate of Meredith Kercher, the British student who was murdered in the Italian town of Perugia, broke down and confessed to being involved with two men in her death, it emerged yesterday.

Detectives said that Amanda Knox, 20, an American student who had the bedroom next to Miss Kercher,confessed to a criminal act. Ms Knox’s boyfriend, Raffaele Sollecito, 23, from Bari, southern Italy, and Patrick Diya Lumumba, 37, of Perugia, are also being held.

Detectives said that they believed that Ms Kercher may have been killed after refusing to take part in a violent orgy. Police have said that they now consider the case to be cut and dried.

The body of Ms Kercher, 21, from Coulsdon, South London, was discovered last Friday under a duvet in her bedroom at the house she shared with Ms Knox. She was partially clothed and her throat had been cut, it is believed, with a penknife.

Police said that the breakthrough came when Ms Knox, from Seattle, “crumbled” under questioning and confessed to what had happened late on Thursday and early on Friday. Pathologists have put the time of death as between midnight and 2am.

All three suspects were taken to Perugia police headquarters early yesterday, and last night were in custody on suspicion of involvement in murder and sexual violence.

There were reports last night that Mr Lumumba, whose wife is Polish, had claimed to be the grandson of Patrice Lumumba, the revolutionary who became the first prime minister of the Republic of Congo in 1960. He was assassinated in 1961.

Arturo De Felice, the chief of police in Perugia, said that charges could follow, adding that the inquiry into Ms Kercher’s murder was “concluded”. It had become clear that the murder of Ms Kercher, a student at Leeds University who was studying Italian at the University for Foreigners, in Perugia, was sexually motivated.

“All three [suspects] took part in the act,” he said. Ms Kercher had been “morally upright”, Mr De Felice added, and no traces of drugs or alcohol had been found in her blood. “She was a victim, nothing more,” he said. A post-mortem examination found evidence of sexual activity before Ms Kercher’s death. Medical examiners refused to confirm reports that she had been raped.

Ms Knox, an exchange student, studies German and Italian at the University of Washington. Last month she wrote on a social networking website: “I’ve been working every night (except for Monday night) from 10pm to 2.30am at a bar called Le Chic. It’s a really small place owned by this man from the Congo. His name is Patrick.”

In July Ms Knox, who was educated at a $12,000-a-year (£5,750) Jesuit college, was fined $269 over a “residential disturbance”. She wrote on Facebook: “I don’t get embarrassed and therefore have very few social inhibitions.”

Mr Sollecito, a computer-science student, had told a British Sunday newspaper last weekend that he and Ms Knox had been horrified to see Ms Kercher’s body after police broke down her door. He said that he and Ms Knox had reported an apparent burglary and had been concerned to find Ms Kercher’s door locked from the inside. “It was hard to tell it was Meredith at first but Amanda started crying and screaming. I dragged her away because I didn’t want her to see it, it was so horrible,” he said.

Last night Mr Sollecito’s father said that he could not believe that his son was involved in any way.

Mr Lumumba moved to Italy in 1988, police said. He works at a bar in the town and holds an official resident’s permit. His friends expressed incredulity at his alleged involvement. One, a bar owner, said that he was “a cultivated man, a great guy, a reggae musician and the father of a little boy”. He was seen on Monday at the torchlit vigil held in Ms Kercher’s memory.

Giuliano Amato, the Italian Interior Minister, described the murder as “a terrible business in which people that were in this young woman’s home tried to have relations with her she didn’t want to have, and she was killed”.

Last night Ms Kercher’s sister, Stephanie, said that Meredith had intended to come home this weekend “with a suitcase laden with chocolates” for her mother’s birthday.

She added: “Meredith touched the lives of everyone she met with her infectious, upbeat personality, smile and sense of humour.”

Amanda Knox described on MySpace how she found the house in which Meredith Kercher was murdered:

“We run into a very skinny girl who looks a little older than me putting up a page with her number on the outer wall of the university. I chat it up with her, she speaks English really well, and we go immediately to her place, literally two minutes from my university. It's a cute house that is right in the middle of this random garden in the middle of Perugia. Around us are apartment buildings, but we enter through a gate and there it is. I’m in love. I meet her roommate Molly. The house has a kitchen, two bathrooms, and four bedrooms. Not to mention a washing machine, and internet access. Not to mention, she owns two guitars and wants to play with me. Not to mention the view is amazing. Not to mention I have a terrace that looks over the city/countryside. I put down a down payment. I’m feeling sky high. These girls are awesome. Really sweet, really down to earth, funny as hell. Neither are students . . . they are desperate for roommates because the two they wanted disappear[ed] all of a sudden"

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