Italian prosecutors on Tuesday asked for a 30-year prison sentence for former US student Amanda Knox at a retrial where she is being judged in absentia for the murder of her housemate, Italian media reported. Prosecutors at the trial also requested that her ex-boyfriend Raffaele Sollecito be given 26 years, saying the punishment should be harsher for Knox because she initially accused someone else of the crime. Knox and Sollecito have already served four years in prison for the murder of British student Meredith Kercher, who was found half-naked in a pool of blood on November 2, 2007 in the house she shared with Knox. The pair, who have always protested their innocence, were acquitted on appeal in 2011 but the supreme court earlier this year overturned that ruling, sending the case back to the appeals stage at a court in Florence. The retrial is expected to reach a verdict in January. Knox is now back in the United States and experts say it is highly unlikely that she could ever be extradited even if she is convicted and even if that conviction is upheld in another appeal to the supreme court. Rudy Guede, a local petty thief and drug dealer, has been convicted separately and is serving a 16-year sentence for the gruesome murder which shocked the university town of Perugia and has divided opinions. Prosecutor Alessandro Crini told the court on Tuesday that the DNA evidence, which is highly disputed in the case, showed that Sollecito and Knox had stabbed Kercher while Guede sexually assaulted her. Crini said he believed Knox had used a large kitchen knife that was later found by investigators in Sollecito's home, pointing to faint DNA traces of Kercher found on the blade and of Knox on the handle. He said the sexual element became "marginal" as the drug-fuelled violence increased. "They were trying to get rid of someone who had to be shut up," Crini said. Knox during one interrogation accused Patrick Lumumba, the owner of a bar where she worked as a waitress, of being in the house at the time of the murder. But Crini said a phone wiretap had overheard Knox telling her mother that Lumumba was not there. "What gave her the certainty if not the fact that she was there?" Crini asked, saying that the young woman's version of what happened had "little credibility". Crini said Knox claimed she had returned to the house after sleeping at Sollecito's when Kercher was already dead but claimed she had not noticed anything and even showered in a bathroom dirty with Kercher's blood. He also said Knox's initial and later retracted statement to police that she had heard "a scream and violence" in the house "contained elements of truth". "Where do these elements come from if not having been directly involved in the incident?" he said. Crini also said that Sollecito's claim that he spent most of the evening at home on his computer was not backed up by analysis of the computer. Criticising the previous appeals court ruling that acquitted Knox and Sollecito, Crini said it had made the mistake of "isolating" individual pieces of evidence without looking at the full picture. The supreme court had "wiped out" that ruling, he said.