The military commander of the Libyan rebels fighting to topple Col Muammar Gaddafi has been killed, the rebel National Transitional Council says.
NTC head Mustafa Abdul-Jalil said Gen Abdel Fattah Younes was killed by assailants, and the head of the group responsible had been arrested.
He said Gen Younes was summoned for questioning about military operations, but never made it to the meeting.
Reports said Gen Younes was suspected of ties to pro-Gaddafi forces.
"With all sadness, I inform you of the passing of Abdel Fattah Younes, the commander-in-chief of our rebel forces," Mr Jalil announced late on Thursday. "The person who carried out the assassination was captured."
Mr Jalil did not elaborate on the identity or motivations of the assailants. It is not clear where the attack took place.
Correspondents say Mr Younes was not trusted by all the rebels because of his previous role in cracking down on dissidents.
Two aides to Gen Younes, Col Muhammad Khamis and Nasir al-Madhkur, were also killed in the attack, Mr Jalil said, adding that there would be three days of mourning in their honour.
Gen Younes is a former Libyan interior minister who defected to the rebel side in February.
He was also part of the group that helped bring Col Gaddafi to power in 1969.
Some unconfirmed reports said Gen Younes and two aides had been arrested earlier on Thursday near Libya's eastern front.
Shortly after the announcement of Gen Younes' death, gunmen entered the grounds of the hotel in the eastern city of Benghazi where Mr Jalil was speaking, reportedly firing into the air before being convinced to leave.