ROME — Italy’s highest criminal court ordered a whole new trial for Amanda Knox and her former Italian boyfriend on Tuesday, overturning their acquittals in the gruesome slaying of her British roommate.
The move extended a prolonged legal battle that has become a cause celebre in the United States and raised a host of questions about how the next phase of Italian justice would play out.
Knox, now a 25-year-old University of Washington student in Seattle, called the decision by the Rome-based Court of Cassation ‘‘painful’’ but said she was confident that she would be exonerated.
The American left Italy a free woman after her 2011 acquittal — but only after serving nearly four years of a 26-year prison sentence from a lower court that convicted her of murdering Meredith Kercher.
The 21-year-old British exchange student’s body was found in November 2007 in a pool of blood in the bedroom of a rented house the two shared in the Italian university town of Perugia. Her throat had been slit.
Raffaele Sollecito, Knox’s Italian boyfriend at the time, was also convicted, sentenced, and later acquitted.
It could be months before a date is set for a fresh appeals court trial for Knox and Sollecito in Florence, which was chosen because Perugia has only one appellate court.
Italian law cannot compel Knox to return for the new trial and one of her lawyers, Carlo Dalla Vedova, said she had no plans to do so.
‘‘She thought that the nightmare was over,’’ Dalla Vedova told reporters on the steps of the courthouse. But “she’s ready to fight.’’
He spoke minutes after relaying the top court’s decision to Knox by phone shortly after 2 a.m. local time in Seattle.
Another Knox defender, Luciano Ghirga, was gearing up psychologically for his client’s third trial.
Ghirga said he told Knox: ‘‘You have always been our strength. We rose up again after the first-level convictions. We’ll have the same resoluteness, the same energy’’ in the new trial.
Still, it was a tough blow for the former exchange student, whose parents have had to mortgage both their homes to raise funds for her lengthy, expensive defense.
‘‘It was painful to receive the news that the Italian Supreme Court decided to send my case back for revision when the prosecution’s theory of my involvement in Meredith’s murder has been repeatedly revealed to be completely unfounded and unfair,’’ Knox said in a statement.
Prosecutors alleged that Kercher was the victim of a drug-fueled sex game gone awry. Knox and Sollecito denied wrongdoing and said they weren’t even in the apartment that night, although they acknowledged they had smoked marijuana and their memories were clouded.
An Ivory Coast man, Rudy Guede, was convicted of the Kercher slaying in a separate proceeding and is serving a 16-year sentence.
Sollecito, whose 29th birthday was Tuesday, sounded shaken when a reporter reached him by phone.
‘‘Now, I can’t say anything,’’ said the Italian, who has been studying computer science in the northern city of Verona after finishing up an earlier degree while in prison.
For those familiar with the US legal principle of ‘‘double jeopardy’’ — by which no one who is acquitted of a crime can be tried again for it — the idea that Italian justice system allows prosecutors to appeal acquittals is hard to absorb.
Knox attorney Dalla Vedova dismissed the ‘‘double jeopardy’’ concern, insisting the high court ruling Tuesday hadn’t decided anything about the defendants’ guilt or innocence, but merely ordered a fresh appeals trial.
Dalla Vedova said Knox wouldn’t come to Italy but would follow the case from home. He said he didn’t think the new appeals trial would begin before early 2014.