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

Knox called the decision "painful" but said she was confident that she would be exonerated.

Italian law cannot compel Knox to return for the new trial, and her lawyer said she had no plans to do so. The appellate court hearing the new case could declare her in contempt of court but that carries no additional penalties.

Italy's Court of Cassation ruled that an appeals court in Florence must re-hear the case against the American student and her former Italian boyfriend for the murder of 21-year-old Meredith Kercher. The exact issues that have to be reconsidered won't be known until the court releases its full ruling within 90 days.

Knox, now a student at the University of Washington, stayed up until 2 a.m. Seattle time to hear her fate and issued a statement through a family spokesman.

"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," she said.

Knox 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.

Knox, now 25, and Raffaele Sollecito, who turned 29 on Tuesday, were arrested shortly after Kercher's body was found in a pool of blood in November 2007 in her bedroom. Kercher, whose throat had been slashed, had shared an apartment with Knox and others in Perugia, an Italian university town where the two women were exchange students.

Prosecutors alleged 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 slaying in a separate proceeding and is serving a 16-year sentence. Knox and Sollecito were also initially convicted of the murder and given long prison sentences, but were then acquitted on appeal and released in 2011.

The high court's ruling Tuesday overturned the appeals court acquittals.

"She thought the nightmare was over," Knox attorney Carlo Dalla Vedova said after the decision was released.

The court on Tuesday also upheld a slander conviction against Knox. During a 14-hour police interrogation, Knox had accused a local Perugia pub owner of carrying out the killing. The man was held for two weeks based on her allegations, but was then released for lack of evidence.

Dalla Vedova said Knox wouldn't come to Italy "for the moment" but would follow the case from home. He said he didn't think the new appeals trial would begin before early 2014.

It is unclear what would happen if Knox 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," Dalla Vedova said Monday.

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

The appeals court that acquitted Knox and Sollecito 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.

It's not clear what part of the appeals sentence was faulted by the high court in ordering a new trial.

Kercher's family attorney, Francesco Maresca, said after Tuesday's ruling: "Yes, this is what we wanted."

Sollecito's attorney, Giulia Bongiorno, noted that Tuesday's ruling was not a determination of guilt but merely a need for further study of the appeals court ruling.

"It's a decision that cancels a verdict and orders a retrial," she said. "I'm not concerned about a deeper reading of the documentation, because I know the documentation."

She acknowledged that perhaps the appeals court ruling had been "too generous" in ruling that the pair simply did not commit the crime, but was confident that Sollecito's innocence would be affirmed.

In her statement, Knox took the Perugia prosecutors to task, saying they "must be made to answer" for the discrepancies in the case. She said "my heart goes out to" Kercher's family.

After nearly four years behind bars in Italy, Knox returned to her hometown of Seattle after the 2011 acquittal and Sollecito resumed his computer science studies, following the degree he earned while studying in prison.

Italy's judicial system allows for two levels of appeals, and prosecutors can appeal acquittals.

Although the court on Monday heard gruesome details, including how Kercher choked on her own blood, it wasn't ruling on the guilt or innocence of the defendants. Its sole task was to decide if the appellate trial was properly conducted.

Dalla Vedova had argued Monday that the slander verdict against Knox should be thrown out because she was questioned without a lawyer even though police essentially treated the student as a suspect in their 14-hour interrogation session.

Because of time she served in prison before the appeals-level acquittals, Knox didn't have to serve time for the slander conviction. Text of statement issued by Amanda Knox

Here is the statement issued by Amanda Knox after Italy's highest criminal court overturned her acquittal in the slaying of her British roommate Meredith Kercher. The statement was issued Tuesday by Knox family spokesman David Marriott.

"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. I believe that any questions as to my innocence must be examined by an objective investigation and a capable prosecution. The prosecution responsible for the many discrepancies in their work must be made to answer for them, for Raffaele's sake, my sake, and most especially for the sake of Meredith's family. Our hearts go out to them. 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." —

Key dates in the Amanda Knox case

Key dates in the case of American student Amanda Knox, convicted in the slaying of British roommate Meredith Kercher.

— Nov. 2, 2007: The body of Kercher, 21, is found in her apartment in Perugia, Italy. Investigators say she was killed the night before.

— Nov. 6, 2007: Knox is arrested with then-boyfriend Raffaele Sollecito and Diya "Patrick" Lumumba, the Congolese owner of pub where Knox occasionally worked.

— Nov. 20, 2007: Lumumba, implicated by Knox statements to police, is released from jail for lack of evidence.

— Dec. 6, 2007: Ivory Coast national Rudy Hermann Guede is extradited from Germany and jailed on arrival in Italy.

— Oct. 28, 2008: Judge indicts Knox and Sollecito on murder and sexual assault charges. Guede, who was granted a fast-track trial, is convicted of murder and sexual assault and sentenced to 30 years in prison.

— Jan. 16, 2009: Trial of Knox and Sollecito opens in Perugia.

— June 12, 2009: Knox takes stand; tells court she was shocked by Kercher's death, offers alibi, says police beat her into making false statement.

— Dec. 4, 2009: Court finds Knox guilty of murder and sexual assault, sentences her to 26 years in prison. Sollecito is convicted of same charges and sentenced to 25 years.

— Dec. 22, 2009: Appeals court upholds Guede's conviction but cuts sentence to 16 years.

— Nov. 24, 2010: Appeals trial for Knox and Sollecito opens in Perugia.

— Dec. 16, 2010: Italy's highest criminal court upholds Guede's conviction and 16-year prison sentence.

— June 29, 2011: Independent forensic report ordered by the appeals court finds much of the DNA evidence used to convict Knox and Sollecito is unreliable.

— Oct. 3, 2011: Appeals court clears Knox, Sollecito of murder convictions, orders them freed immediately

— March 26, 2013: Italy's highest criminal court overturns acquittal of Knox and Sollecito, orders new trial.