* U.S. student convicted then acquitted of Briton's murder
* Top court has ordered retrial of Knox and ex-boyfriend
* Says judges had underestimated evidence against them
Italy's top court said on Tuesday it had ordered a retrial of American Amanda Knox and her ex-boyfriend in the murder of British student Meredith Kercher because their acquittals contained "shortcomings, contradictions and inconsistencies."
Knox and Italian Raffaele Sollecito were initially found guilty of killing the 21-year-old Leeds University student in 2007 during what was described as a drug-fuelled sexual assault, but both were cleared on appeal in 2011.
In March of this year, Italy's top appeals court overturned the acquittals and ordered a retrial of a sensational murder case that has prompted harsh criticism of the Italian justice system.
Explaining its reasons on Tuesday, the Court of Cassation said the judges in the appeals case had underestimated the evidence against the two accused, assessing clues one by one and not stepping back to view the big picture.
The one person still in jail for the murder, Ivory Coast-born Rudy Guede who is serving a 16-year sentence, may not have committed the crime alone, and the possibility that Kercher was killed during a group sex game would need to be re-examined, the Court of Cassation said.
Kercher's body was found with more than 40 wounds, including a deep gash in the throat, in the apartment she shared with Knox in Perugia, in central Italy, where both were studying during a year abroad.
Knox, now 25 and living back in the United States after four years in an Italian jail, has consistently denied involvement in her roommate's death.
Sollecito's lawyer, Giulio Bongiorno, said: "If there was an erotic game then there needs to be a search for the other people involved, who are certainly not Raffaele Sollecito and Amanda Knox," news agency Ansa reported.
SHOCK IN SEATTLE
Kercher's lawyers, who have described the acquittal as "contradictory and illogical," have welcomed the plan for a retrial.
In Knox's hometown of Seattle, attorney Anne Bremner, spokeswoman for the group Friends of Amanda, said she was stunned by news that the retrial ruling included reference to a ritualistic sex allegation.
Knox, dubbed "Foxy Knoxy" in many early media reports, was initially portrayed as promiscuous and dishonest but a lobbying campaign by her family helped change perceptions.
"I was really surprised," Bremner said of the ruling. "The prosecutor abandoned (that allegation) back in the trial phase. ... I thought it was the weakest part of the evidence."
She added that there was nothing in the case to support such a claim.
"There is no evidence - hair, fiber, fingerprints, nothing," she said. "It just doesn't make any sense. It's a fantastic allegation."
Bremner, who has handled international extraditions in other cases, said it would take years for the legal wrangling to be concluded.
"The United States could well decide not to extradite her, and Italy could then try her in absentia," Bremner said. "Say she's convicted in absentia, then there could be an appeal to the (Italian) Supreme Court again."
Unlike law in the United States and some other countries, the Italian system does not contain so-called "double jeopardy" provisions that prevent a defendant being tried twice for the same offence.
David Marriott, spokesman for the Knox family, declined to comment, saying the family had not yet received an official translation from their attorney in Rome.