Secretary of State Hillary Rodham Clinton announced on Thursday the creation of a review board led by a veteran diplomat and former under secretary of state, Thomas R. Pickering. She also briefed lawmakers behind closed doors on Capitol Hill. But the State Department now faces Congressional demands for an independent investigation of the attacks and any security failures that might have added to the death toll.

“In my judgment, which is informed by numerous briefings and discussions with experts, the attack in Benghazi was not a black swan,” Senator Susan Collins, Republican of Maine, said at a hearing on Wednesday, “but rather an attack that should have been anticipated, based on the previous attacks against Western targets, the proliferation of dangerous weapons in Libya, the presence of Al Qaeda in that country and the overall threat environment.”

Investigators and intelligence officials are now focusing on the possibility that the attackers were affiliated with, or possibly members of, Al Qaeda in the Islamic Maghreb — a branch that originated in Algeria — or at least in communication with it before or during the initial attack at the mission and the second one at the mission’s annex, a half-mile away.

One extremist now under scrutiny is a former detainee at the prison at Guantánamo Bay, Cuba, Abu Sufian Ibrahim Ahmed Hamouda, a Libyan who is a prominent member of an extremist group called Ansar al-Sharia, which some have blamed for the attack. “It is safe to assume that any significant extremist in eastern Libya is going to be under a lot of scrutiny right now,” an American intelligence official said, adding that it was premature to draw any conclusions.

The most significant inconsistency between Libyan and American accounts is whether the attack that night began with a small protest over the trailer of “The Innocence of Muslims,” parts of which were broadcast on Egyptian television. American officials insist there was a protest that began peacefully, only to be hijacked by armed militants. But Libyan witnesses, including two guards at the building, say the area around the compound was quiet until the attackers arrived, firing their weapons and storming the compound from three sides, beginning at 9:30 p.m. on Sept. 11. A witness said that some of those attacking referred to the film’s insults to Islam.

Matthew G. Olsen, director of the National Counterterrorism Center, said at a Senate hearing on Wednesday that the authorities believe “this was an opportunistic attack” that “evolved and escalated over several hours.”

What is clear, however, is that those who arrived at the mission — not officially a consulate, though Libyans call it that informally — came intending to inflict maximum damage on the building. They quickly overwhelmed a small security detail that included three guards from a force called the 17th of February Brigade and five Libyans employed by a British security company called Blue Mountain.