The US government is investigating whether the attack on the US Consulate in Libya in which America's ambassador was gunned down was a planned terrorist strike to mark the anniversary of the September 11 2001 attacks rather than a spontaneous mob enraged over an anti-Islam YouTube video.
President Barack Obama declared that the US would "work with the Libyan government to bring to justice" those who killed Chris Stevens and three other Americans. It was the first killing of a US ambassador in more than 30 years.
The attack on the Benghazi Consulate was "a planned, co-ordinated, well-executed military style event", House of Representatives Intelligence Committee chairman Mike Rogers said.
Meanwhile, in a show of force, the Pentagon moved two warships to the Libyan coast. Officials said one destroyer, the USS Laboon, moved to a position off the coast on Wednesday and the destroyer USS McFaul was on its way and should be stationed off the coast within days, increasing the number of US Navy destroyers in the Mediterranean from four to five.
Officials said the ships, which carry Tomahawk cruise missiles, do not have a specific mission, but they give commanders flexibility to respond to any mission ordered by the President.
Pentagon spokesman George Little said: "Without commenting on specific ship movements, the United States military regularly takes precautionary steps when potential contingencies might arise in a given situation. That's not only logical in certain circumstances, it's the prudent thing to do."
At the same time, around 50 US Marines headed to Libya to reinforce security at US diplomatic buildings, initially at the American embassy in Tripoli.
White House press secretary Jay Carney said it was too early to judge whether the Benghazi attack was planned. "I know that this is being investigated, and we're working with the Libyan government to investigate the incident. I would not want to speculate on that at this time," he said. Several Libyan security guards were also killed.
The attack in Libya, which came hours after a mob stormed the US embassy in Cairo and tore down the American flag, was presumed to have been triggered by a film, whose trailer has gone viral on YouTube, depicting the Islamic prophet Mohammed in disrespectful ways.
"Make no mistake. Justice will be done," a sombre Mr Obama pledged at the White House, with secretary of state Hillary Clinton at his side. He ordered increased security at US diplomatic missions overseas, particularly in Libya, and said he condemned "in the strongest possible terms the outrageous and shocking" attack.