As the deaths of US diplomats in Libya turned into an election issue here, officials on Wednesday said they suspect Tuesday's attack was a well-planned assault by militants instead of a rampaging mob, renewing concerns about the role of extremists in the wake of the Arab Spring.
The attackers appear to have used protests over an inflammatory film as a pretext to stage a major assault involving small arms fire and rocket-propelled grenades that lasted several hours, overwhelming the security team at the consulate.
"That's the working hypothesis at the moment," said a senior US official, who spoke on condition of anonymity.
"This was a complex attack," he told AFP. "They seemed to have used this (protest) as an opportunity."
The gunmen, who opened fire on the American consulate and kept US security teams at bay for hours, may have had links to al Qaeda or been inspired by the terror network, officials said, stressing that investigators needed more time.
"It is too early to definitively attribute the attack to al Qaeda or its affiliates," the official said.
However, the chairman of the House Intelligence Committee, Mike Rogers, said the attack resembled an al Qaeda operation.
"There are still some fuzzy details... but clearly it has all the hallmarks of an al Qaeda-style event," the Republican lawmaker told CNN.
Initial accounts suggested the assault was the result of violent riots but as more details emerged, officials and experts said the evidence pointed to a deliberate plot staged to mark the 9/11 anniversary.
US moves forces
In response to Tuesday's attack, the US dispatched two naval destroyers to waters off Libya and deployed a 50-strong counter-terrorism team of US Marines to bolster security at its Tripoli embassy.
The United States also has unmanned drone aircraft at its disposal to help Tripoli track militants who may have carried out the attack, US officials have said.