(CBS News) -- Witnesses of last week's deadly attack on a U.S. consulate in Libya have told CBS News that the alleged anti-American protest that U.S. officials say morphed into the assault never actually took place.
The new details add to the widening rift between U.S. and Libyan accounts of the attack as Secretary of State Hillary Clinton is expected to brief members of Congress behind closed doors Thursday. On Wednesday, a top U.S. counterterrorism official told lawmakers on Capitol Hill that the assault was "opportunistic," but a "terrorist attack."
Clinton's briefing comes amid calls from Capitol Hill for more information on the attack, and complaints that the Obama administration has not been forthright.
"I'm just stunned and appalled that there wasn't better security for all of the American personnel at that consulate given the high threat environment," Sen. Susan Collins, R-Maine, said during a congressional hearing.
The attack on the 11th anniversary of the Sept. 11, 2001, terror attacks killed four Americans, including U.S. Ambassador Christopher Stevens. Libyan President Mohamed Magariaf said on CBS' "Face the Nation" Sunday that the execution of the attack showed it was planned far in advance. The Obama administration has called the attack more impulsive than planned.
On Wednesday, Matthew Olsen, director of the National Counterterrorism Center, told senators that the United States doesn't have specific intelligence that the attack was planned in advance, according to the Reuters news agency.
"The best information we have now, the facts that we have now, indicate that this was an opportunistic attack" on the consulate, Olsen said, according to Reuters. "The attack began and evolved and escalated over several hours."
U.S. officials have said since the immediate aftermath of the attack that it originally started as an anti-American protest against an amateurish online video made in the U.S. that insults the Muslim Prophet Muhammad.
"At this point, what I would say is that a number of different elements appear to have been involved in the attack, including individuals connected to militant groups that are prevalent in eastern Libya, particularly in the Benghazi area," Olsen said, according to Reuters. "As well, we are looking at indications that individuals involved in the attack may have had connections to al Qaeda or al Qaeda's affiliates, in particular Al Qaeda in the Islamic Maghreb."
One man the FBI and intelligence agencies are looking at as a possible suspect in the assault is a former prisoner at the U.S. military prison in Guantanamo Bay, Cuba, CBS News senior correspondent John Miller reports.
Sufyan bin Qumu was released from Guantanamo in 2007 to the Libyan regime headed by Muammar Qaddafi. Qaddafi jailed him, but he was then set free on a promise to renounce violence. When the Arab Spring uprising began in Libya, Qumu became part of the rebel movement which eventually toppled Qaddafi.
"He's a great suspect on paper," Miller said. "There is some intel about him but it's way too early to verify these reports we've seen that he is the prime suspect or the prime mover. Way too early."
CBS News correspondent Margaret Brennan reports FBI investigators in Libya haven't yet gone to the destroyed consulate, and Miller cautions it's still too soon to make any firm determinations about the attack.
"Whether it was pre-planned or not is tough because in this environment, in Libya," says Miller. "In a normal place when people show up with weapons and cars and an effective assault, you say that requires pre-operational planning. Libya's a place that went through 13 months of this. There's all kinds of weapons and militias with cars and weapons, so it's the one kind of place this could happen spontaneously."