The US has evacuated all of its personnel from Benghazi to the Libyan capital following the killing of its Ambassador Christopher Stevens and three other embassy staff.
His death and those of his colleagues have also sparked speculation over those responsible.
The four died during a fierce attack on the Benghazi consular building by armed militants on Tuesday night. Their car was hit by a rocket as they tried to leave the compound.
Although the incident was initially thought to have been triggered by anger over a US-made anti-Islamic film, US officials believe it may have been planned and are investigating possible al Qaeda involvement.
In Washington President Barack Obama condemned the killings but insisted they would not harm relations with Libya's new elected government.
"The United States condemns in the strongest terms this outrageous and shocking attack. We're working with the government of Libya to secure our diplomats. I've also directed my administration to increase our security at diplomatic posts around the world... Today we mourn for more Americans who represent the very best of the United States of America. We will not waiver in our commitment to see that justice is done for this terrible act. And make no mistake, justice will be done," said US President Barack Obama.
Video of the storming of the diplomatic compound has been shown on Benghazi TV. The use of extensive weaponry has come as no surprise to US investigators since after the collapse of Muammar Gaddafi's regime last year, Libyan government arsenals were looted, making them available to potential militants.
