Libyan rebels raise their flag in 2011 (Creative Commons)

In a Senate Homeland Security Committee meeting Wednesday, National Counterterrorism Center Director Matt Olsen said the four Americans, including Ambassador Chris Stevens, "were killed in the course of a terrorist attack upon our embassy."

According to Olsen, members of multiple armed militant groups were involved in the events, but information concerning the extent of their participation has not been released.

“The picture that is emerging is one where a number of different individuals were involved, so it’s not necessarily an either/or proposition," he said.

Olsen added that they are currently investigating the possible involvement of al-Qaeda, specifically the Algerian-based al-Qaeda in the Islamic Maghreb.

Last week, White House Press Secretary Jay Carney stated that the "protests were in reaction to a video," referring to the anti-Islamic video The Innocence of Muslims which caused widespread anti-American relatiation throughout the region.

Olsen said the protests provided an "opportunistic" time for the terrorists to strike, though the attack did not appear to be preplanned.

"There was no specific intelligence regarding an imminent attack prior to September 11th on our post in Benghazi," he said, though other officials have disagreed. 

"I just don't think that people come to protests equipped with RPGs and other heavy weapons," said Republican Sen. Susan Collins of Maine. "And the reports of complicity - and they are many - with the Libyan guards who were assigned to guard the consulate also suggest to me that this was premeditated."

According to Reuters, a team of FBI investigators, currently in Tripoli, is headed to Benghazi to conduct an on-site investigation of the attack.

Reach Staff Reporter Greg Asciutto here. Follow him on Twitter.