Last week's deadly assault on a US diplomatic mission in the Libyan city of Benghazi was a "terrorist attack" but probably not a pre-planned operation, a senior US official said on Wednesday.
The attack, which left four Americans including US Ambassador Chris Stevens dead, was carried out on the 11th anniversary of the September 11 attacks and amid protests in the Muslim world against an anti-Islamic film.
In Washington, where President Barack Obama's election rival Mitt Romney has criticized the handling of the attack, there has been keen interest in whether the attackers were simply an angry mob or an organized gang.
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. It evolved and escalated over several hours," he said, emphasizing that a US investigation was continuing.
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 the Islamist militant group Al-Qaeda was among the suspects.
"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," he said.
Benghazi was the cradle of last year's Libyan revolt that ousted strongman Moamer Kadhafi, and the region is still prey to several loosely-organized militia groups with varying degrees of ties to the interim government.
Initial reports into Thursday's attack on the US consulate suggested that it had been attacked by protesters angered by an amateur movie produced by private US Christian groups deemed insulting to the Prophet Mohammed.
But the gang that stormed the compound and torched the consulate was heavily-armed, and Libyan officials have said that a militant group took advantage of the protest to launch a pre-planned attack.