Italy's highest criminal court has overturned the acquittal of Amanda Knox for the murder of British student Meredith Kercher and ordered a new trial.
The court heard six hours of arguments on Monday before going into deliberations and issued its verdict this morning.
Knox has been waiting anxiously in Seattle to hear if her long legal battle is over.
Ms Kercher's family lawyer, Francesco Maresca, said after the ruling: "Yes, this is what we wanted."
Stephanie Kercher, Meredith's sister, said: "There are a lot of unanswered questions still.
"We are very hopeful that it going back to court will help find those answers and find out the truth of what happened that night.
"Whilst we are not happy about going back to court, and it will not bring her back, we have to make sure we have done all we can for her.
"We still have a long journey ahead and we are very grateful for the support of the public and in Italy - we just want justice for Mez."
Knox and co-defendant, Raffaele Sollecito were initially convicted of the murder of British student Meredith Kercher but were then acquitted in 2011.
Knox, now a student at the University of Washington, said: "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 the case 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," she said.
Knox, now 25, and Raffaele Sollecito, who turns 29 today, were arrested in 2007, shortly after Miss Kercher's body was found in a pool of blood in her bedroom in the rented apartment she shared with the American and others in the university town of Perugia, where they were exchange students. Her throat had been slashed.
Prosecutors alleged that Miss Kercher was the victim of a drug-fuelled sexual assault.
Knox and Sollecito have both maintained their innocence, although they said that smoking marijuana the night Miss Kercher was killed had clouded their recollections.
Knox and Sollecito were convicted and given long prison sentences: 26 years for Knox, 25 for Sollecito.
But an appeals court acquitted them in 2011, criticising 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 added that Knox and Sollecito had no motive to kill Miss Kercher.
After nearly four years behind bars in Italy, Knox returned to her hometown of Seattle and Sollecito resumed his computer science studies. Knox is now a student at the University of Washington, according to her family spokesman, Dave Marriott.
In the second and final level of appeal, prosecutors are now seeking to overturn the acquittals, while defence attorneys say they should stand.
The court has remanded the case to a new appeals court trial. Italian law cannot compel Knox to return to Italy. The Italian 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 she is convicted at that trial and if the conviction is upheld by the highest court, then Italy could seek her extradition," Knox's lawyer Carlo Dalla Vedova said.
Italy is not obliged to seek her extradition but it could decide to do so. Then it would be up to the United States to decide if it honours the request. US and Italian authorities could also come to a deal that would keep Knox in the United States.
A young man from the Ivory Coast, Rudy Guede, was convicted of the killing in a separate proceeding and is serving a 16-year sentence. Miss Kercher's family has resisted theories that Guede acted alone.
The lawyer for the Kercher family, Francesco Maresca, said the family was likely to issue a statement when the decision is announced. They did not attend the arguments.
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