Amanda Knox could face the threat of an extradition request from the Italian government after Italy's highest court overturned her acquittal over the murder of Meredith Kercher.
Although Italian law cannot compel Knox to return from the US for the new trial there is an extradition treaty in place between Italy and America.
Signed in 1984 when Ronald Reagan was president, the treaty obligates each country to extradite anyone charged with or convicted of an extraditable offence - or any offences punishable under the laws of both countries by a jail sentence of more than one year.
Italy must provide the US with specific documents to demonstrate they have "probable cause to believe" that Miss Kercher was murdered and Miss Knox committed the offence.
It would then be up to the US to decide if it honours the request. US and Italian authorities could also come to a deal that would keep Knox in America.
If Knox fails to attend the trial the appellate court hearing the case could also declare her in contempt of court - but that carries no additional penalties.
Knox, now 25, and her former boyfriend Raffaele Sollecito, 29 today, were arrested shortly after Miss Kercher's body was found in a pool of blood.
The appeals court that acquitted them in 2011 criticised 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, Knox returned to her hometown of Seattle and Sollecito resumed his computer science studies, following the degree he successfully earned while studying in prison.
Knox is now a student at the University of Washington, according to her family spokesman, Dave Marriott.
Italy's judicial system allows for two levels of appeals, and prosecutors can appeal acquittals.
Although the court yesterday heard gruesome details, including how Miss Kercher choked on her own blood, it was not ruling on the guilt or innocence of the defendants. Its sole task was to decide if the appellate trial was properly conducted.
Details of today's ruling issued are not expected to be issued for several weeks.