Intelligence sources believe the deadly attack on an American consulate in Libya was directly linked to al Qaeda, with a former Guantanamo inmate involved, according to US media reports.
Officials had revealed the connection on the same day a top Obama administration official called the assault in Benghazi a “terrorist attack”, after it had previously been described as a spontaneous act.
America’s ambassador to Libya, Chris Stevens, and three other US officials died as rocket-propelled grenades were fired into the compound on the 11th anniversary of the September 11 attacks.
A number of arrests were made after the attack, which appeared to have been an act of vengeance by an angry mob of Muslims reacting to an anti-Islamic film made in the US.
In the aftermath of the attack President Barack Obama vowed to “bring to justice” those responsible.
The US military moved two navy destroyers towards the Libyan coast in what officials described as a move to give the administration flexibility for any future action against Libyan targets.
On Wednesday Matt Olson, the director of America’s National Counterterrorism Centre, said at a Senate hearing: “Yes, they were killed in the course of a terrorist attack on our embassy.”
He said it appeared to still be an “opportunistic” attack that had escalated over several hours and that al Qaeda was among the suspects.
Mr Olsen said there was no specific intelligence about “significant advanced planning or co-ordination”.
The gang that stormed the compound was reported to be heavily armed, and Libyan officials have said that a militant group took advantage of the protest to launch a pre-planned attack.