Prosecutors in South Africa are seeking to overturn the verdict and sentence handed to Oscar Pistorius, with the case likely to be heard in November.
They have filed an appeal calling for the Paralympian star to be convicted of murder for killing his girlfriend, just days before he is due to be released from prison.
Pistorius has spent less than a year behind bars since being convicted in a sensational trial on the lesser charge of culpable homicide, and is expected to leave jail on Friday (local time) for a form of house arrest.
But the 28-year-old could soon be back in prison if prosecutors are successful in their argument in the appeal court that he should have been convicted of murder rather than culpable homicide, a charge equivalent to manslaughter.
If they win their case, which is expected to be heard in November, Pistorius could face at least 15 years in jail.
"I think the chances are pretty good that the appeals court will rule in favour of the state and overturn the verdict," said Ulrich Roux, a criminal lawyer in Johannesburg.
"He is faced with the unusual circumstance that he's released on house arrest and then the court could find him guilty of murder and he'll have to return to prison."
Pistorius does not dispute that he shot model and law graduate Reeva Steenkamp four times through a locked toilet door in his Pretoria home in the early hours of the Valentine's Day in 2013.
But he said he mistook the 29-year-old for an intruder. Prosecutors insist that he deliberately killed her after an argument.
They are appealing both the judgment and the sentence, calling the five-year jail term "shockingly light".
Prosecutors argue in the appeal papers that even based on the athlete's own version of events, Pistorius "knew there was a person behind the closed door" and "deliberately fired shots into the door".
"The only conceivable finding ... should be that he intended to kill the person in the cubicle," they say, adding that Pistorius' evidence "can never be found to be reasonably possible".
The athlete's defence team has until September 17 to file its response.
South African correctional services officials have indicated that Pistorius has been a good inmate and qualifies for house arrest, a routine procedure in South Africa.
"He's not out on parole on the 21st August, he's having his sentence converted to a house arrest sentence," said David Dadic, a criminal lawyer based in Johannesburg.
"He's now confined to a house for a period until he's actually on parole," said Dadic. "They'll confine him essentially to what he would be doing in prison, but in the confines of his own house."