A senior US official said that the last week’s attack on the American consulate in the Libyan city of Benghazi was a "terrorist attack" but probably not a pre-planned operation.
There has been interest in Washington in whether the attackers, who killed four US consulate officials, including US Ambassador Chris Stevens, were simply angry crowd or an organized group.
On Wednesday, the director of the US government's National Counterterrorism Center told lawmakers that, while many questions remain to be answered, he was prepared to describe the killings as "a terrorist attack."
But the director, Matthew Olsen, immediately qualified that statement.
"The best information we have now, the facts that we have now indicate that this was an opportunistic attack on our embassy," he told the Senate Homeland Security Committee, under questioning from Senator Joe Lieberman.
"The attack began and evolved and escalated over several hours at our embassy -- our diplomatic post in Benghazi," he said, emphasizing that a US investigation was continuing.
Fox News meanwhile cited "intelligence sources" as saying that former Guantanamo detainee Sufyan Ben Qumu -- transferred to his native Libya in 2007 and released the following year -- was involved in the attack.
"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," he said.
"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," the global network's North Africa franchise.
US Secretary of State Hillary Clinton was due to address lawmakers on Thursday at a closed door secret intelligence briefing, but in the meantime Olsen confirmed that Al-Qaeda was among the suspects.