A witnessed parked near the crime scene where Amanda Knox's roommate Meredith Kercher was murdered testified Tuesday that he heard no screams or anything unusual that night.

Pasqualino Coletta said his car broke down, forcing him to wait for help between 10:30 p.m. and 11 p.m. on November 1, 20007, the night 21-year-old Kercher was killled.

Knox and her former boyfriend Raffaele Sollecito are on trial in Perugia, central Italy, on charges of murder and sexual violence for Kercher's death. Both deny wrongdoing.

Autopsy results and accounts by Kercher's friends puts her time of death between 9 and 11 p.m. that night. Her body was found in a pool of blood the next day and prosecutors allege that Kercher was killed during what had begun as a sex game.

A witness for the prosecution whose apartment overlooks Kercher and Knox's, said in court last March that she heard a scream that night, but later said she wasn't sure of the date.

Meanwhile, a coroner, Francesco Introna, told a jury Saturday that a stab wound to the neck of Kercher was caused by a shorter knife than the one believed to be the murder weapon, lawyers said. Introna also said that no more than a single attacker could have assaulted the victim on the night of the 2007 slaying.

Introna, who was called to the stand by Sollecito's defense lawyers, testified that the cut on Kercher's neck was made with a knife with a 3-3 1/2-inch-long (8-9 centimeters long) blade, according to a lawyer representing the Kercher family, Francesco Maresca.

Prosecutors say that a 6 1/2-inch (16.5 centimeters) knife found at Sollecito's house matched the victim's wounds and could be the murder weapon. They say the knife had Kercher's DNA on the blade and Knox's on the handle.

Introna said that the bedroom where Kercher was killed was too small and that it would be "physically impossible" that three people could have attacked her, Maresca said. Helped by two assistants, he staged an attack to show how Kercher might have been assaulted from behind by only one person.

However, when cross-examined by prosecutors, Introna clarified he has never been to the house where Kercher was slain and used data collected by forensic experts to determine the size of the bedroom.

Maresca noted that when the court went to inspect the scene of the crime in April, six or seven people could fit into Kercher's bedroom.

Prosecutors allege that Sollecito held Kercher by the shoulders from behind while Knox touched her with the point of a knife. Guede tried to sexually assault Kercher and then Knox fatally stabbed her in the throat, according to the prosecutors.

A third person, Rudy Hermann Guede of the Ivory Coast, was convicted in a separate trial last year and sentenced to 30 years in prison. He denies wrongdoing and has appealed his conviction.