DNA traces which do not match any of the three official suspects have been found on the bra of Meredith Kercher, the British student murdered in Perugia last November, suggesting that more people may have taken part in the attack than thought.

Forensic scientists working on the case also said they had revised the time of Ms Kercher's death, previously put at between 10pm and midnight on November 1, to between 9pm and 4am on November 2.

Traces of the DNA of Raffaele Sollecito and Rudy Hermann Guede, two of the suspects currently being held in prison, have allegedly already been found on Ms Kercher's torn and bloodstained bra.

However, police said that two new traces found did not come from either man; nor did they match the DNA of Amanda Knox, Ms Kercher's American flatmate, who is also in prison pending charges.

Police said they were trying to establish the identity of the two new people whose DNA had been found, and were keeping “an open mind” about how the traces got there. Ms Kercher was found semi-naked, with her throat cut, on the morning of November 2, in her bedroom at the whitewashed cottage she shared with Ms Knox and two female Italian students.

Prosecutors have said the post-mortem examination showed signs of sexual violence, suggesting she had been killed in a drugs-fuelled sex game. However, Il Messaggero, the Rome daily, reported that the pathologist who carried out the exam believed the sexual violence “may have been simulated since no bruising consistent with a rape” was found on Ms Kercher's body. Investigators said Ms Kercher's attackers had also simulated a break-in by smashing a window.

Il Messaggero said the new time of death would appear to undermine the alibi of Diya “Patrick” Lumumba, the Congolese bar owner who was accused by Ms Knox of being the murderer but was released after being able to prove he was at his bar until 3am. No trace of his DNA has been found at the murder scene.

In an interview with ABC News, Amanda Knox's parents, Edda Mellas and Curt Knox, and her 19-year-old sister, Deanna, have criticised the “media frenzy” over Ms Knox's alleged involvement in the murder, saying the picture painted of her as “the girl from Seattle with an angel face and ice-cold eyes” who ran wild in Italy was “unrecognisable”.

Ms Knox has repeatedly altered her version of events, first declaring she was at Mr Sollecito's flat on the night of the murder, but then admitting she was at the cottage and had covered her ears when she heard Ms Kercher's screams. She later reverted to her original story.

However, her mother told ABC that her daughter had been interrogated without lawyers or interpreters while in a frightened state. “This was a horrible crime, but I couldn't understand why immediately Amanda was painted in this horrible light, where she was unrecognisable,” she said.

Ms Knox's sister said: “Amanda is the kindest person I know. She will do anything to make people happy and she cares about everyone else before herself.”

Ms Knox's parents have visited her twice a week for the past three months, either together or separately. She and the other suspects can be held for up to a year before being charged.

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