Italy’s highest criminal court on Tuesday overturned the acquittal of Amanda Knox in the slaying of her British roommate and ordered a new trial, prolonging a case that has become a cause célèbre in the United States.

The Court of Cassation ruled that an appeals court in Florence must re-hear the case against the American and her Italian-ex-boyfriend for the murder of 21-year-old Meredith Kercher.

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The exact issues that have to be reconsidered won’t be known until the court releases its full ruling.

Lawyers for Knox and her ex-boyfriend Raffaele Sollecito looked grim as they huddled with prosecutors and court officials to get details after the ruling was issued. Lawyers for the Kercher family said they had got what they wanted.

Kercher’s body was found in November 2007, in her bedroom of the house she shared with Ms. Knox and others in Perugia, an Italian university town where the two women were exchange students. Her throat had been slashed.

Prosecutors alleged Kercher was the victim of a drug-fuelled sex game gone awry. Ms. Knox and Mr. Sollecito denied wrongdoing and said they weren’t even in the apartment that night, though they acknowledged they had smoked marijuana and their memories were clouded.

An Ivory Coast man, Rudy Guede, was convicted of the slaying in a separate proceeding and is serving a 16-year sentence. Ms. Knox and Mr. Sollecito were also initially convicted of the murder and given long prison sentences, but were then acquitted on appeal and released.

The high court’s ruling on Tuesday overturns the appeals court acquittals.

Italian law cannot compel Ms. Knox to return from the U.S. for the new trial. The appellate court hearing the case could declare her in contempt of court but that carries no additional penalties.

It is unclear what would happen if she was convicted in a new appeals trial.

“If the court orders another trial, if she is convicted at that trial and if the conviction is upheld by the highest court, then Italy could seek her extradition,” Ms. Knox’s lawyer Carlo Dalla Vedova said on Monday.

It would then be up to the United States to decide if it honours the request. U.S. and Italian authorities could also come to a deal that would keep Ms. Knox in the United States.

Ms. Knox, now 25, and Mr. Sollecito, who turned 29 today, were arrested shortly after Kercher’s body was found in a pool of blood.

The appeals court that acquitted them in 2011, criticized virtually the entire case mounted by prosecutors. The appellate court noted that the murder weapon was never found, said that DNA tests were faulty and that prosecutors provided no murder motive.

After nearly four years behind bars in Italy, Ms. Knox returned to her hometown of Seattle and Mr. Sollecito resumed his computer science studies, following the degree he successfully earned while studying in prison.

Ms. Knox is now a student at the University of Washington, according to her family spokesman, Dave Marriott.

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