Italian prosecutors have launched an appeal against the acquittals of Amanda Knox and Raffaele Sollecito for the 2007 murder of Briton Meredith Kercher.
The pair's convictions for murdering Miss Kercher in Perugia were overturned in appeal court last year.
A jury cleared American Miss Knox and her Italian ex-boyfriend after doubts about the gathering of DNA evidence.
Miss Kercher, 21, from Croydon, south London, had been found semi-naked in her bedroom and with her throat cut.
She had also been sexually assaulted, leading prosecutors to believe she was killed in a brutal sex game that went wrong.
Ivory Coast national Rudy Guede was convicted in a separate trial of sexually assaulting and stabbing Kercher and sentenced to 16 years in prison.
Miss Knox and Mr Sollecito had been facing 26-year and 25-year jail terms respectively following their initial convictions in 2009.
They had spent nearly four years in jail when they were cleared and released last October.
Much of the case in the appeal court centred on DNA evidence on a knife, thought to be the murder weapon, which a review suggested could be flawed.
Prosecution evidence of Miss Knox's DNA on the handle, and Miss Kercher's DNA on the blade of the kitchen knife, which was found at Mr Sollecito's flat, had been key to the original convictions.
But the independent review cast doubt over those DNA traces due to concerns about poor procedures in evidence collection and forensic testing and the possibility of contamination.
An eight-member appeal court jury, comprising two judges and six members of the public, overturned the original verdicts.
Prosecutors said at the time that they would appeal against the acquittals.
Miss Kercher had been on a year abroad from Leeds University and was sharing a cottage in the central Italian hilltop town of Perugia with Miss Knox, who is originally from Seattle.