ROME (AP) -- It's not over yet for Amanda Knox.
Italy's top criminal court dealt a stunning setback Tuesday to the 25-year-old college student, overturning her acquittal in the grisly murder of her British roommate and ordering her to stand trial again.
"She thought that the nightmare was over," Knox's attorney, Carlo Dalla Vedova, told reporters minutes after conveying the unexpected turn of events to his client, who had stayed up to hear the ruling, which came shortly after 2 a.m. West Coast time. "But she's ready to fight."
Now a student at the University of Washington in Seattle, Knox 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 October 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 exchange student's body was found in a pool of blood, her throat slit, in a bedroom of the house the two shared in Perugia, a university town 100 miles north of Rome.
Raffaele Sollecito, Knox's Italian boyfriend at the time, was also convicted of the Nov. 1, 2007, murder, then later acquitted. His acquittal was also thrown out Tuesday and a new trial ordered.
Italian law cannot compel Knox to return for the new trial and Dalla Vedova said she had no plans to do so.
In any case, the judicial saga is likely to continue for years. It will be months before a date is set for the new trial, to be held in Florence instead of Perugia because the small town has only one appellate court, which already acquitted her.
Prosecution and defense teams must also await details of the ruling explaining why the high court concluded there were procedural errors in the trial that acquitted Knox and Sollecito. The court has 90 days to issue its explanation.
Another Knox defender, Luciano Ghirga, said she was gearing up psychologically for her 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 mortgaged 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.
She said the matter must now be examined by "an objective investigation and a capable prosecution."
"No matter what happens, my family and I will face this continuing legal battle as we always have, confident in the truth and with our heads held high in the face of wrongful accusations and unreasonable adversity," Knox said.
Prosecutors alleged that Kercher was the victim of a drug-fueled sex game gone awry. Knox, then 20, and Sollecito, then 24, 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 slaying in a separate trial 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 an earlier degree while in prison.
Later, Sollecito said in a statement that he was "saddened" by the high court decision and will "continue to fight for my innocence, hopeful and confident that truth will prevail."
A local Italian news report quoted Sollecito's current girlfriend as saying he and Knox spoke by phone and described him as being psychologically destroyed.
His lawyer, Luca Maori, said neither Sollecito nor Knox ran any danger of being arrested. "It's not as if the lower-court convictions are revived," he said, noting that the high court didn't determine "whether the two were innocent or guilty."
For those familiar with the U.S. legal principle of "double jeopardy" -- which holds that no one acquitted of a crime can be tried again for it -- the idea that the Italian justice system allows prosecutors to appeal acquittals is hard to absorb.
Knox attorney Dalla Vedova dismissed the "double jeopardy" concern, maintaining the high court ruling hadn't decided the defendants' guilt or innocence, but merely ordered a fresh appeals trial, which he said was unlikely to start before early 2014.