The BBC has seen evidence that a man was tortured and killed in detention, after clashes between rival militia groups in Libya at the weekend.
Members of Izzedine al-Ghool's family say he was accused of siding with the regime of the late Muammar Gaddafi.
Former rebel commanders say they have opened a new front to disarm Gaddafi loyalists holding out around Gharyan, 80km (50 miles) south of Tripoli.
At least nine people were killed in last week's fighting, officials say.
Interim Prime Minister Abdurrahim al-Keib and Defence Minister Osama al-Juweili visited the area in an effort to halt the violence.
The fighting is said to have erupted when members of the Gharyan militia went to arrest people suspected of having ties to the former Gaddafi regime in the town of Assabia, about 16km (10 miles) to the south-west.
Assabia's militia reportedly refused to hand the suspects over, triggering clashes in which rockets and heavy machine-guns were fired.
The BBC's Gabriel Gatehouse, in Tripoli, reports that Mr Ghool's family say he was the commander of an armed brigade in his hometown of Assabia, and that he was detained on Friday after the fighting began.
But they deny that he remained loyal to the former regime.
His body is currently in cold storage at a mortuary in the capital.
The BBC has seen marks on his legs and buttocks that suggest he was subjected to brutal treatment.
Our correspondent says this is an acutely sensitive subject for the interim authorities. A military officer at the mortuary denied the BBC permission to interview the doctor who conducted the autopsy.
But a nurse who was present when the body was examined said Mr Ghool had been subjected to electric shocks and savagely beaten.
On Sunday, a Gharyan military commander said his men had taken a number of prisoners, who have since been released.
A senior member of the Gharyan council told the BBC that the claims of torture would be investigated and that any perpetrators would be brought to justice.
But he admitted that officials were often powerless to control the militias in Gharyan, who have yet to hand in their weapons nearly three months after the official end of fighting.
Earlier this month, Libya's interim leader warned of the risk of civil war if militias were not disarmed.