A couple held on suspicion of murdering the British exchange student Meredith Kercher bought risqué lingerie and discussed plans for “wild sex” the day after Ms Kercher was found dead, it was claimed yesterday.

A shopkeeper said that he overheard Amanda Knox, 20, an American, and her Italian boyfriend Raffaele Sollecito, 23, discussing their plans on the evening of November 3, less than 48 hours after Ms Kercher, 21, was found semi-naked with her throat cut.

Italian newspapers reported that Carlo Maria Scotto di Rinaldi, 30, the owner of a clothes shop in Perugia, had testified to police that he clearly remembered Ms Knox and Mr Sollecito coming into his shop. The couple were also recorded on the store’s CCTV cameras kissing. Mr di Rinaldi said that they stayed for half an hour, and that Ms Knox bought two thongs. He overheard Mr Sollecito say to Ms Knox: “You can put these on at home and we’ll have wild sex.” He said that they spoke in English, “but I know the language and had no difficulty in understanding”.

Yesterday Mr Sollecito asked to make a statement to investigators for the first time. He has remained silent since he, Ms Knox and Diya “Patrick” Lumumba, a Congolese bar owner, were taken to a prison outside Perugia on November 6.

Detectives have pieced together a new possible reconstruction in which Rudy Hermann Guede, an alleged drug dealer arrested this week in Germany, and Ms Knox took part in the assault.

Mr Sollecito’s role remains unclear. The presumed murder weapon, a kitchen knife found at his flat, has Ms Kercher’s and Ms Knox’s DNA on it but not his. A bloody footprint in Ms Kercher’s bedroom is “compatible” with his Nike trainers, and bloody rags and sponges apparently used to clean up the crime scene were also found at his flat.

Before the arrests Mr Sollecito told police that Ms Knox had been with him the whole night of November 1.

He later backtracked, however, telling police that Ms Knox had been absent from his flat from 9pm until 1am. He said that Ms Knox had persuaded him to lie on her behalf.

Italian police said that DNA taken from Mr Guede’s toothbrush proved that he had “violent and incomplete sex” with Ms Kercher on the night that she died. Scientific tests had confirmed that the bloody fingerprint on her pillow corresponded to Mr Guede’s, and that his DNA matched samples taken from Ms Kercher’s body. Mr Guede, who is in German police custody awaiting extradition to Italy, has admitted that he was there and had sex with Ms Kercher, but says that he did not kill her. German police said that he would be returned to Italy between December 10 and 14.

Police said that phone records showed that Mr Guede and Ms Knox had talked on their mobile phones several times before and after Ms Kercher’s death.

Italian police suspect that Mr Guede killed Ms Kercher. Claudia Matteini, the investigating judge, said: “There are, without doubt, grave indications of the guilt of Guede, especially in the light of precise and irrefutable analysis carried out by the forensic police.”

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