Evidence suggests that three people actively participated in a "group attack" on the British exchange student Meredith Kercher, an Italian judge said.

Referring to the three suspects in the murder of the 21-year-old, who was found dead last month with her throat slashed, the judge said that “none of them can be said to have played a passive role”.

Investigators believe that Kercher’s American flatmate, Amanda Knox, Knox’s Italian boyfriend, Raffaele Sollecito, and a third suspect, Rudy Hermann Guede, had all been involved in the killing on November 2.

Read more of this report at the Times of London

While all three have insisted that they played no part in the murder, police have pointed out apparent contradictions in their testimonies.

Judges in Perugia, where Kercher’s body was found, yesterday explained their decision to reject an appeal by Guede against his continued detention, saying that his testimony was "full of decisive falsehoods."

Judge Maurizio Bufali said the evidence suggested that not only had all three had been in Kercher’s house on the night, but there was a “quick departure of all of them after the tragic conclusion of the evening”.

There must have been a "powerful reason" why the suspects — who were all, like Kercher, in their early twenties — had “taken such a cruel crime to such extreme limits”, the judges said. Witnesses have said that there were tensions between Knox and Kercher on a number of issues, including the alleged theft by Knox of cash from Kercher to pay for drugs.

The judges said that Guede’s fingerprints were on Kercher’s pillow and his DNA had been found in the lavatory of her house. They said that they found his claim that he had been in the bathroom when Kercher was murdered by an unknown Italian assailant implausible.

The judge made his comments after it emerged that a Harry Potter book that Knox claimed to have read at her boyfriend’s flat on the evening of the murder had been found at the cottage where Kercher was killed. Knox, who has given several different accounts of her movements on November 1 and 2, told Giuliano Mignini, the chief investigating magistrate, on Monday that she had spent the evening and night at Sollecito’s flat.

She said they had smoked cannabis, watched a film, made love, and "read a few pages" of the Harry Potter book. However, police conducting a renewed search of the cottage that Knox shared with Kercher and two Italian female students found the book there on Tuesday, Italian newspapers reported yesterday. La Stampa said that it was “another blow to the credibility of their alibi”. Police said they had also found blood-stained tissues and a knife in the undergrowth. However, they said that the knife was not the murder weapon, since it was not sharp enough to have caused the wounds in Kercher’s neck.

Knox’s footprint was also found during the new search on a postcard on the floor of the room used by one of the Italian flatmates — who was not there at the time of the murder — where the window had been broken. Police suspect the window was smashed to simulate a break-in. Knox had sworn in testimony that she did not enter the room.

Police also removed “bloodstained items of clothing” apparently overlooked in previous searches. Defence lawyers were not allowed to attend the search but watched it on closed-circuit television from a police van outside the cottage.