Mohammed Karah died today of a gunshot wound to the head, after participating in an offensive against gangs who had been stealing cars and blocking roads in the Rishfana area on the outskirts of Tripoli.
Two others from the security forces were wounded.
The sources spoke on the condition of anonymity because they were not authorized to brief journalists.
Since the fall of dictator Muammar Gaddafi in Libya's 2011 civil war, gunmen have killed low-level government employees, activists, clerics and security officials.