Defence lawyers in the Meredith Kercher murder trial in Italy have sought to assert that Ivory Coast drifter Rudy Guede acted alone in murdering the British student.
Earlier they had raised questions over the credibility of the prosecution claims on the weapon allegedly used in the crime.
Their witness, Francesco Introna, a coroner, testified last week that the cut on Miss Kercher's neck was made with a knife with a blade 3-3.5ins long and not compatible with the 6.5in kitchen knife produced as the murder weapon.
Police say the knife was recovered from the house of Raffaele Sollecito, an Italian engineering student and a former boyfriend of US student Amanda Knox, both of whom are the accused in the second phase of the murder trial. .
Further damaging the prosecution case, Dr. Introna also told the court in Perugia that Kercher perhaps was killed by a lone attacker.
Dr Introna said it would have been "physically impossible" for three people to have attacked Miss Kercher in her tiny bedroom. The room was not large enough for "four people to interact, as proposed by the prosecution," he said.
The prosecution have also said the knife had small traces of Miss Kercher's DNA on the blade and Miss Knox's on the handle.
But Miss Knox's lawyers and family have argued ever since she was arrested that there is no conclusive proof that the kitchen knife was the murder weapon.
They say it was natural that the handle would bear traces of Miss Knox's DNA because she had used it to chop food while making dinner at her then boyfriend's house.
Just two months into the medieval city, on Nov.2, 2007, Meredith Kercher a European Studies student from south London, was found semi-naked, with her throat slashed in a hillside cottage she shared with Ms Amanda Knox of the US and two Italian women. Her body bore more than 40 injuries.
In the days following the murder, prosecutors arrested Amanda Knox, 21, her former boyfriend and Italian Raffaele Sollecito, 24, and Rudy Guede, 21, a drifter and immigrant from the Ivory Coast. Sollecito is an IT student from a wealthy southern Italian family.
It was a case of drug-fuelled sex orgy gone awry, it has been claimed. The prosecution says that Sollecito had pinned her down while Miss Knox touched her with the point of a knife and Guede sexually assaulted her. Knox then allegedly stabbed the young British woman in the throat.
"When I closed my eyes, I could only see red," Guede wrote in an account for the police after his arrest. "I have never seen so much blood. All of that blood on her beautiful face."
Ms Knox herself told police that she had covered her ears in the kitchen to block out Ms Kercher's screams, but she has since denied all such statements made to the police.
Last year the African was found guilty of murder and sentenced to 30 years in jail.
But Judge Paolo Micheli ruled that there was enough evidence to send Knox and Sollecito to trial on charges of murder and aggravated sexual assault.
Sollecito and Knox have also been accused of simulating a break-in by smashing a window and theft of cash and credit cards.
Separately Guede ha also begun his appeal, claiming Ms Knox had killed Ms Kercher in a row over stolen cash.
Defence lawyers maintain that Guede, a small-time drug dealer, was involved in at least two previous burglaries and often carried a knife, the defence said.
In Sept 2007 - two months before the murder - a burglar later identified as Guede had broken into a house and threatened a couple with a knife before running away.
Paolo Brocchi, a local solicitor, described how, in Oct 2007, an intruder used a rock to smash the window of his office, clamber through it and steal a mobile phone and a laptop computer.
Police arrested Guede a few days later after he broke into a school in Milan, armed with a knife, and found the stolen goods in his possession.
"The break-in was a carbon-copy of the way he used a rock to break a window at Kercher's house and enter," said Luca Maori, a lawyer representing Sollecito.
"It helps show how Guede entered alone that night, probably to steal, found Kercher undressing, tried to rape her and then killed her."
When Guede broke into the lawyer's office, he climbed up to a window which was about 15ft from the ground. When police investigated the apparent break-in at the cottage Miss Kercher and Miss Knox shared outside Perugia's stone walls, they found that a window of a similar height had been smashed. They decided that it was too high to clamber through and instead came up with the theory that it had been broken from inside by Miss Knox and Mr Sollecito to fake a burglary and throw police off their tracks.
"We heard evidence today that Rudy was perfectly able, and accustomed, to scaling such heights," said Luciano Ghirga, a lawyer representing Miss Knox.
The trial continues.