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 Barack Obama declared in a White House appearance that the U.S. would “work with the Libyan government to bring to justice” those who killed Ambassador Chris Stevens and three other Americans.

Intelligence officers said the attack on the Benghazi consulate was “too coordinated or professional to be spontaneous,” according to a U.S. counterterrorism official.

The official spoke on condition of anonymity because he was not authorized to discuss the incident publicly.

White House press secretary Jay Carney said it was too early to judge whether the 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 also were killed.

The FBI was sending evidence teams to Libya, said a law enforcement official.

Analysts are working on several different scenarios based on intelligence that could lead to a motive for the attack.

Some concern the possibility of targeting high-ranking officials, according to a law enforcement official briefed on the investigation. But none of the intelligence has suggested terrorists would specifically target Stevens, said the official who also spoke on condition of anonymity because he was not authorized to speak publicly about the investigation.

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 Muhammad 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 evening.

“Make no mistake. Justice will be done,” a somber Obama pledged at the White House, 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 U.S. had helped liberate from dictator Moammar Gadhafi during the Arab Spring revolution in Libya last year.

Three Americans were wounded, U.S. officials said.

The aftermath of the two attacks also stirred the U.S. presidential campaign, where until Wednesday, foreign policy had taken a back seat to the struggling economy.

The Republican nominee, Mitt Romney, criticized the administration for statements issued before and after the Cairo attacks that expressed sympathy for those insulted by the video.

“I also believe the administration was wrong to stand by a statement sympathizing with those who had breached our embassy in Egypt instead of condemning their actions,” Romney told a morning news conference. “It’s never too early for the United States government to condemn attacks on Americans and to defend our values.”

Obama said later than Romney “seems to have a tendency to shoot first and aim later.”

Obama and Clinton made a rare joint visit to the State Department, where grieving colleagues of Stevens and the other three Americans killed in Benghazi 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. Flags had already been lowered in many places to commemorate the victims of the 9/11 attacks.

Clinton denounced those who might kill over an insulting movie.

“There is no justification for this,” Clinton said. “None. Violence like this is no way to honor religion or faith and as long as there are those who would take innocent life in the name of God, the world will never know a true and lasting peace.”

Underscoring the administration’s frustration, Clinton wondered aloud about the attack in Benghazi, which Gadhafi had once threatened to destroy.

“This is not easy,” she said. “Today, many Americans are asking, indeed I asked myself, how could this happen? How could this happen in a country we helped liberate, in a city we helped save from destruction? This question reflects just how complicated and, at times, just how confounding, the world can be.”

“But we must be clear-eyed in our grief,” she said, saying the attack was carried out by a “small and savage group” not representative of the Libyan people. She noted that Libyan security guards had tried to fight off the attackers, had carried Stevens’ body to the hospital and led other consulate employees to safety.

Stevens, a 52-year-old career diplomat, was killed after he became separated from other American officials during the consulate 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.

Stevens is the first U.S. ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan.

Three other Americans were also killed and the State Department identified one of them as Sean Smith, an Air Force veteran who had worked as an information management officer for 10 years in posts such as Brussels, Baghdad and Pretoria. Smith was also well-known in the video game community.

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.

U.S. officials said some 50 Marines were being sent to Libya to reinforce security at U.S. diplomatic facilities.

Stevens 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.