WASHINGTON - The Obama administration, roiled by the first killing of a U.S. ambassador in more than 30 years, is investigating whether the assault on the U.S. Consulate in Libya was a planned terrorist strike to mark the anniversary of the Sept. 11, 2001, attacks and not a spontaneous mob enraged over an anti-Islam YouTube video.

President Obama declared in a White House appearance that the United States would "work with the Libyan government to bring to justice" those who killed Ambassador J. Christopher Stevens and three others. "These four Americans stood up for freedom and human dignity," he said.

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 Wednesday, and the destroyer USS McFaul was en route and should be stationed off the coast within days, increasing the number of Navy destroyers in the Mediterranean from four to five.

The Pentagon also sent about 50 Marines from Spain to reinforce security at U.S. diplomatic facilities, initially at the U.S. Embassy in Tripoli, the capital. The FBI and CIA were sending teams to Libya, a law enforcement official said.

The two-hour attack on the Benghazi consulate was "a planned, coordinated, well-executed military style event," said House Intelligence Committee Chairman Mike Rogers, R-Mich.

He said U.S. intelligence had not yet determined who was responsible but said, "It wasn't some folks who had some guns in their garage and said let's shoot up the consulate."

The attack in Libya, which came hours after a mob stormed the U.S. Embassy in Cairo and tore down the U.S. flag, was presumed to have been triggered by a movie, whose trailer has gone viral on YouTube, depicting the Islamic prophet Mohammed in disrespectful ways. In an extraordinary move, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, called anti-Islamic preacher Terry Jones and asked him to stop promoting the film. A spokeswoman said the church would not show the film Wednesday.

While the protesters in Cairo appeared to be genuinely outraged over the video, the attackers in Benghazi were armed with mortars and rocket-propelled grenades. Officials said it was possible that an organized group had been waiting for an opportunity to exploit the protests over the video or perhaps even generated the protests as a cover for their attack.

White House press secretary Jay Carney said it was too early to judge whether it was planned. "I know that this is being investigated," he said.

Analysts are working on several scenarios based on intelligence that could lead to a motive for the attack. Some concern the possibility of targeting high-ranking officials, a law-enforcement official said. But none of the intelligence has suggested terrorists would specifically target Stevens, the official said.

"Make no mistake. Justice will be done," a somber Obama pledged at the White House, with Secretary of State Hillary Rodham Clinton at his side.

He ordered increased security at U.S. diplomatic missions overseas, particularly in Libya, and said he condemned "in the strongest possible terms the outrageous and shocking" attack. Clinton said she was particularly appalled that the attack took place in Benghazi, which the United States had helped liberate from dictator Moammar Gadhafi during the Arab Spring revolution in Libya last year.

Three Americans were wounded and several Libyan security guards were killed, U.S. officials said.

Obama and Clinton made a rare joint visit to the State Department, where grieving colleagues of Stevens and the three others gathered in a courtyard. The president also ordered U.S. flags to be flown at half-staff at government and military buildings and vessels around the world until sunset on Sept. 16.

Clinton denounced those who might kill over an insulting movie. "There is no justification for this," she said. "None."

Stevens was a 52-year-old career diplomat, well-liked by officials in the new Libyan government. He spoke Arabic and French and had already served two tours in Libya, including running the office in Benghazi during the revolt against Gadhafi. He was confirmed as ambassador to Libya by the Senate earlier this year.

He was killed after he became separated from other American officials during the attack. It's unclear when he died: He was taken by Libyans to a hospital, and his remains were delivered hours later to U.S. officials at the Benghazi airport.

There were unconfirmed reports that Stevens was pursued by Islamic militants to his death in a safe house, where he may have died of asphyxiation from smoke in a grenade explosion. He was the sixth U.S. ambassador to be killed on duty, and the first killed abroad since 1979, when Ambassador Adolph Dubs was killed in Afghanistan.

Of the three others killed, the State Department identified one as Sean Smith, an Air Force veteran who had worked as an information management officer for 10 years. The identities of the others were being withheld pending notification of relatives.

"The mission that drew Chris and Sean and their colleagues to Libya is both noble and necessary, and we and the people of Libya honor their memory by carrying it forward," Clinton said.

The New York Times contributed to this report.