A Harry Potter book which Amanda Knox, the American student suspected of involvement in the murder of Meredith Kercher, claimed to have read at her boyfriend's flat on the evening of the murder has been found — not at the flat but at the cottage where the British student was killed.

Ms Knox, who has given several different accounts of her movements between November 1 and 2, told Giuliano Mignini, the chief investigating magistrate, on Monday that she had spent the entire evening and night at the flat of Raffaele Sollecito, her Italian boyfriend.

She said that they had smoked cannabis, watched a film, made love, and “read a few pages” of one of the Harry Potter series of novels by J.K. Rowling, “in the German edition”. Ms Knox was a language student, and Mr Sollecito, who studied computer science, had spent time in Germany under the Erasmus student exchange programme.

However, police conducting a renewed search of the whitewashed cottage that Ms Knox shared with Ms Kercher and two Italian women students found the Harry Potter volume there yesterday, Italian newspapers reported this morning.

La Stampa said that while, in theory, Ms Knox and Mr Sollecito could have brought the book with them on the morning of November 2, when they arrived at the cottage to find — by their own account — Ms Kercher's bedroom door locked and a window smashed, this was unlikely.

The Harry Potter clue was “another blow to the credibility of their alibi” the paper said.

Police said that they had also found bloodstained tissues and a butter knife in the undergrowth. The knife was not the murder weapon because it had a rounded end and was not sharp enough to have caused the wounds in Ms Kercher's neck, police said. The presumed murder weapon is a kitchen knife found in Mr Sollecito's flat with Ms Knox's and Ms Kercher's DNA on it. Mr Sollecito's lawyers have contested this theory and had asked for a search for “the real murder weapon” to be carried out in undergrowth around the cottage.

The new search also discovered a footprint belonging to Ms Knox on a postcard found on the floor of the room used by one of the Italian flatmates (absent at the time of the murder), where the window had been broken. Police suspect the window was smashed to simulate a break in. Ms Knox has sworn in testimony that she did not enter the room in question.

Reports said that the imprint of a Nike trainer found near Ms Kercher's body and matching trainers worn by Mr Sollecito had deteriorated to the point where it was almost invisible, a month and a half after the murder. However, police said that they had detailed photographs of the imprint and full laboratory reports of tests conducted at the cottage after the crime. In the search yesterday they also found the bloodstained imprint of a tiny star matching a star on the sole of one of Mr Sollecito's trainers.

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

Judges in Perugia who last week rejected an appeal by Rudy Hermann Guede, the third suspect in the case, against his continued detention said today that they had done so because there was clear evidence to suggest that he, Ms Knox and Mr Sollecito had all taken part in a “group attack” on Ms Kercher in which “none of them can be said to have played a passive role”.

Outlining their reasoning, as required by Italian law, the judges said that forensic scientific evidence suggested that all three had been present at the cottage on the evening of the murder. There was still no motive for the attack on Ms Kercher but there must have been a “powerful reason” why the suspects — who were all, like Ms Kercher, in their early twenties — had “taken such a cruel crime to such extreme limits”.

Witnesses have said that there were tensions between Ms Knox and Ms Kercher on a number of issues, including an alleged theft by Ms Knox of money from Ms Kercher to pay for drugs. Even if this were true it remains unclear why this should have led to a sexual assault on Ms Kercher and her murder.

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

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