DNA tests show the man most recently arrested in connection with the murder of Meredith Kercher had sex with her on the night she died, say Italian police.
Miss Kercher was an exchange student in the Italian city of Perugia
Rudy Guede, 20, denied killing her but has reportedly admitted being in the house at the time.
Mr Guede claimed he heard the doorbell ring and Miss Kercher scream before he confronted and fought "a young guy".
Miss Kercher, 21, a student from Leeds University, was murdered in Perugia, Italy, on 1 November.
Mr Guede, who is originally from the Ivory Coast, was arrested on a train in Germany this week after failing to buy a ticket and is being held while extradition proceedings take place.
He is now a main suspect in the case.
His fingerprints had already been found on a pillow in the bedroom where Miss Kercher, from Coulsdon, south London, was found, and in the bathroom.
Police knew whoever had used the toilet on the night of the murder was the same person who had had sex with Miss Kercher.
Mr Guede has protested his innocence, insisting she was killed by a stranger.
But Italian press reports say police have intercepted internet messages in which he had confessed to being in the house at the time.
He reportedly wrote that Miss Kercher invited him to her house that night.
"After sex I went to the bathroom. The doorbell rang and then I heard Meredith screaming," he said.
"I fought with a young guy. I was hurt as well but I cannot remember his face," he added.
Mr Guede is expected to be extradited back to Italy within days.
Rudy Hermann Guede was held after fare-dodging in Germany
He faces charges over the murder and sexual assault of Miss Kercher, found with her throat cut in the central Italian city.
One of three others being held over the murder, Congolese bar owner Diya "Patrick" Lumumba, 38, was released from custody by Italian police earlier this week because of lack of evidence.
But he remains a suspect in the student's death.
Miss Kercher's American roommate, Amanda Marie Knox, 20, and her Italian boyfriend Raffaele Sollecito, 23, are still being held.
Investigators say they can now prove Mr Sollecito was not online at the time of the murder as he claimed and that his computer was idle until the early hours of the morning.
It has also emerged that, in a statement given before she was arrested, Amanda Knox said she had noticed blood on her boyfriend's hand.
"I thought it was blood from the fish that he cooked," she said.
Both deny involvement, but under Italian law can be held for up to a year.
Police do not believe the murder was premeditated but allege each of the suspects played a role.
Prosecutors believe Miss Kercher was killed after refusing to take part in an extreme sex game.