The amateurish film, Innocence of Muslims, portrayed the Prophet Mohammad in a negative light. The maker of the controversial film — a Californian real estate developer Sam Bacile, who described himself as an Israeli Jew — has since gone into hiding.
US pastor Terry Jones, who had inflamed anger in the Muslim world in 2010 with plans to burn the Quran, said he had promoted the film.
"While the United States rejects efforts to denigrate the religious beliefs of others, we must all unequivocally oppose the kind of senseless violence that took the lives of these public servants," said President Barack Obama.
A US Marine counterterrorism team has been dispatched to Benghazi, reflecting suspicions that a terrorist group may have used the rioters as a shield to target Stevens. The ambassador was supposedly killed by a rocket-propelled grenade -- a weapon normally used by a trained soldier.
The killings have raised questions about the stability of the Libyan regime, which has struggled to contain scores of local militias, and whether more protests might erupt in the Muslim world over the film.