Human Rights Watch has said that Libya's former ambassador to France, Omar Brebesh, died in the custody of a militia from possible torture.
The New York-based HRW said a Tripoli-based militia from the town of Zintan detained Mr Brebesh on 19 January and he died less than 24 hours later.
It said that a preliminary autopsy found the cause of death included "multiple bodily injuries and fractured ribs."
"Photos of Brebesh's body, seen by Human Rights Watch, show welts, cuts and the apparent removal of toenails, indicating that he was tortured prior to death," the rights watchdog said in a statement.
It said a report by the judicial police in Tripoli also found that Mr Brebesh had died from torture and that an unnamed suspect had confessed to killing him.
Militias made up of former rebels who fought Muammar Gaddafi's forces have been facing growing criticism from rights groups for allegedly torturing prisoners, most of whom are loyalists of the slain dictator.
Last week, Amnesty International and Doctors Without Borders reported "widespread torture" in prisons under the control of militias and of some officially recognised military.
The two groups said such cases were observed in the prisons of Tripoli, Misrata and smaller towns such as Ghariyan.
Doctors Without Borders suspended its work in Misrata over these allegations.
Libya's Justice Minister Ali Hamida Ashur has said the authorities would investigate all cases of "torture" in prisons and bring to justice those involved in it.