Amanda Knox has spoken of her anguish after Italy’s highest criminal court overturned her acquittal for the murder of British student Meredith Kercher.

The American returned to her home in Seattle after she was dramatically cleared following four years in jail in 2011, but now faces the prospect of returning to the country for a new trial.

She said prosecutors should be made to answer for apparent “discrepancies” in their case and called for an “objective investigation” into questions surrounding her alleged involvement in the killing.

The body of Leeds University student Miss Kercher, 21, was found in November 2007 in her bedroom at the house she shared with Miss Knox in the Italian university town of Perugia. Her throat had been slashed.

Prosecutors claimed Miss Kercher, from Coulsdon, Surrey, was the victim of a drug-fuelled sex game gone awry.

Miss Knox, now 25, and her Italian ex-boyfriend Raffaele Sollecito, 29, denied wrongdoing. They were convicted following a high-profile trial but were released on appeal and Miss Knox was immediately flown back to America.

She 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. 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.”

Miss Kercher’s older sister Stephanie Kercher, speaking at her Surrey home, told the Press Association: “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.”

Italian law cannot compel Knox to return to the country for a fresh trial. She could be found in contempt of court but that carries no additional penalties. It is unclear what would happen if she was convicted following a new appeal.

But the latest developments mean Miss Knox could face the threat of an extradition request from the Italian government. The US and Italy brought an extradition treaty into force 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.