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. Corker said post-revolutionary Libya was in turmoil, crippled by a "lack of institutions," which was hampering Washington's efforts to investigate the attack with the Libyan government. "I don't think there's been much coordination [between the countries] at all," Corker told the Post. "My sense is that almost everything the American government knows about the situation is what the American government has derived on their own." Asked if he had believed the perpetrators would be brought to justice, he said, "Anybody who's seen even a glimpse of this would have to say that it's going to be very difficult." He spoke a day before White House chief counter-terrorism adviser John O. Brennan was to visit Tripoli for meetings with senior-level Libyan officials. Brennan was to meet Tuesday with Parliament President Mohammed Magarief, who has been de facto head of state since August, as well as Libyan defense, intelligence, interior and foreign ministry officials, officials said. But he will not meet with the Cabinet-level ministers because they were rejected Sunday by Libya's Parliament. The Parliament, known as the General National Congress, ousted newly elected Prime Minister Mustafa Abushagur after rejecting his proposed Cabinet. With his dismissal, Libya now also lacks defense and interior ministers, responsible for apprehending the consulate attackers, The New York Times reported. U.S. Lt. Col. Andrew Wood, a former head of the U.S. Army Special Forces security team in charge of diplomatic security in Libya, told CBS News he and Stevens feared for the diplomatic corps' safety several months before the Benghazi attack. But he said they were rebuffed by the Defense and State departments when he and Stevens requested tighter security. The State Department said it was cutting back, not adding security, he told the network. "The requests were being modified to say, 'Don't even request for DoD support,'" Woods said, referring to the Department of Defense. The State Department removed three security teams and Woods' squad over six months, he said. The State Department told CBS the security-support withdrawal had "no impact whatsoever on the total number of fully trained American security personnel in Libya overall or in Benghazi specifically." Woods is to testify Wednesday before the House Oversight and Government Reform Committee, led by Rep. Darrell Issa, R-Calif.