US suspects Libyan attack was a well-planned assault

US suspects that the deadly attack on its mission in Benghazi in Libya could be a well- planned assault by Islamist hard liners rather than a rampage by a frenzied mob, officials said, warning that investigations underway could change the circumstances of the killings.

Still grappling to unravel the facts about the killing of its ambassador Chris Stevens and three other Consulate Staff almost 24 hours after the incident, US officials said the attack was "complex" and "they seem to have used this (protest) as an opportunity".

The attackers appeared to have used protest over a controversial film as a pretext to stage a major assault using small arms and rockets to overrun the Consulate, they said.

A senior administration official in a background briefing conceded that they did not have much details about the circumstances leading to the death of the ambassador, but admitted that prior to storming of the Consulate there had been heavy exchange of fire between sections of the protesters and the security personnel guarding the compound.

However, the Chairman of the Congress House Intelligence Committee Mike Rogers minced no words in saying that the attack that killed US ambassador and his staff had all the hallmarks of an al-Qaeda operation.

"Details may still be fuzzy... but clearly it has all the hallmarks of an al-Qaeda operation," the Republican lawmaker told CNN.

Tuesday's attack on the American Consulate came amid a wave of protests in Muslim countries against a film made by an Israeli-American director which is deemed offensive to Islam.

US officials today said that at least three Americans were wounded in the attack that killed the envoy.

Following the attack, Washington has ordered evacuation of all its staff in Benghazi to Libyan capital of Tripoli.

The final hours of Stevens where still shrouded in mystery after he was trapped in a blazing inferno in the mission. The FBI has now opened an inquiry into the attack and the circumstances leading to death of the four Americans.