U.S. officials say an assault that killed the U.S. ambassador to Libya and three other Americans in the Libyan city of Benghazi may have been planned rather than being a spontaneous outburst of anti-American sentiment.

The officials said Tuesday's attacks by an armed mob on Benghazi's U.S. consulate and a nearby safe house appeared to be a coordinated and complex operation. But, the officials said it was too early to identify those who killed U.S. Ambassador J. Christopher Stevens and three colleagues or whether the assault was timed to coincide with the 11th anniversary of the September 11 terrorist attacks on the United States.

U.S. State Department officials said the assault began Tuesday at around 10pm local time, with assailants opening fire on the U.S. consulate before torching the building while Ambassador Stevens and U.S. information management officer Sean Smith were inside. They said U.S. security personnel at the consulate became separated from the diplomats due to heavy smoke but later found Smith's body.

Libyan authorities located Stevens and brought him to a Benghazi hospital where a doctor pronounced him dead of smoke inhalation.

U.S. officials said the assault resumed after American and Libyan security forces evacuated other consulate staff to a nearby annex. They said assailants began firing at the building at around midnight, killing two more U.S. personnel, whose names have not been released.

U.S. President Barack Obama condemned the killing of the four Americans as “outrageous and shocking.” Speaking Wednesday at the White House Rose Garden with Secretary of State Hillary Clinton by his side, he vowed to work with the Libyan government to bring the attackers to justice. A U.S. official said the Navy was sending two destroyers toward the Libyan coast as a precautionary measure.

Libyan Interim President Mohammed el-Megarif apologized to the United States for the attack.

U.S. authorities evacuated all consular staff and the bodies of the four Americans from Benghazi to the Libyan capital Tripoli early Wednesday. U.S. officials said 50 Marines were sent to Libya to boost security for the remaining U.S. personnel.

Initial reports said the Benghazi mob was angered by an apparently U.S.-made amateur film that mocks the Prophet Muhammad. Hours before the consulate assault, an Egyptian mob also outraged by the film stormed the U.S. embassy in Cairo and tore down the American flag.

In his Rose Garden address, President Obama said the United States “reject(s) all efforts to denigrate the religious beliefs of others,” a reference to the controversial film.

U.S. Joint Chiefs of Staff Chairman General Martin Dempsey made an unusual phone call to an anti-Muslim American pastor Wednesday, urging him to “consider withdrawing” support for the film. Pastor Terry Jones has presided over a burning of the Quran, an act that triggered Muslim outrage. U.S. military officials were concerned that any promotion of the film by Jones could provoke attacks on U.S. forces in Afghanistan.

A trailer for the film about the Prophet Muhammad was posted on YouTube in July. An Arabic-language translation of the video began circulating in the Middle East in recent days.

Mr. Obama also ordered increased security at U.S. diplomatic posts around the world. After his address, he went to the State Department to console diplomatic staff.

Ambassador Stevens is the first U.S. envoy to be killed on duty in more than 30 years. He was a career foreign service officer and one of the most experienced American diplomats in the region. He had taken up his post in the Libyan capital, Tripoli, in May. U.S. officials said he was making a routine visit to Benghazi at the time of the assault.

Stevens was widely admired by Libyan rebels for his support of their uprising that overthrew longtime Libyan dictator Moammar Gadhafi last year. Secretary Clinton said Stevens “risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation.” She also said the relationship between the United States and Libya will not be “another casualty” of the attack.