U.S. agents are studying security videos from the U.S. Consulate in Benghazi for clues to the attack that killed four Americans, a U.S. senator in Libya said. The investigators are examining video from security cameras at the primary Benghazi compound to help them reconstruct what happened in the Sept. 11 attack and identify attack participants, Senate Foreign Relations Committee member Bob Corker, R-Tenn., told The Washington Post in Tripoli, where he met with Libyan officials. The 5-hour attack on the unmarked mission and nearby annex with rocket-propelled grenades, hand grenades, AK-47 and FN F2000 NATO assault rifles, gun trucks and mortars killed Ambassador to Libya Christopher Stevens, Foreign Service Information Management Officer Sean Smith and private security employees and former U.S. Navy SEALs Glen Doherty and Tyrone Woods. Leaders of Ansar al-Sharia, an extremist Islamist militia group that witnesses said they saw mounting the attack, are still at large. The group is believed linked to al-Qaida in the Islamic Maghreb -- an affiliate of the international terrorist group with origins in Algeria -- and led by former Guantanamo Bay detention camp prisoner Sufyan Ben Qumu. Qumu, a 53-year-old Libyan citizen once considered a \"threat\" to the United States, was released by the Bush administration to Libya Sept. 28, 2007, on the condition Libya keep him in jail. The Gadhafi regime later freed him.