The security officer then went back inside and located Smith’s body in what one U.S. official described as “a heroic effort.” He was unable to find Stevens because the diplomat had been taken to the hospital by “unknown personnel,” the U.S. official continued.

Meanwhile, State Department and Libyan security personnel counterattacked, drove the assailants from the compound, rounded up other American personnel and moved them into a walled annex, the U.S. officials said. The annex came under fire around midnight, and two unidentified Americans were killed. Three others were injured.

The Americans eventually were evacuated from Benghazi, and all but emergency staff left the U.S. Embassy in Tripoli under the protection of the 50-strong unit from the Marines’ Fleet Anti-Terrorist Security Team, an elite contingent dispersed around the world to rapidly respond to terrorist incidents, the U.S. officials said.

Initial reports said the assault began as a protest timed to coincide with the anniversary of the Sept. 11, 2001, attacks and linked to anger over clips posted on the Internet of a U.S.-made film parodying the Prophet Muhammad. But Bishari’s statements and those of the senior administration officials suggested that there was no such protest in Benghazi.

Obama condemned the film, saying that Americans "reject all efforts to denigrate the religious beliefs of others."

"But there is absolutely no justification to this kind of senseless violence. None," he said.

Which organization precisely was behind the attack in Benghazi was unclear amid suggestions that al Qaida may have played a role. Ansar al Sharia, the group whose flag the Benghazi attackers displayed, is one of the largest armed extremist groups operating in Libya. But the attack came just hours after al Qaida leader Ayman al Zawahiri in a video appeal urged Libyans to attack U.S. targets to avenge the killing by a U.S. drone in June of his Libyan second in command, Abu Yahya al-Libi.

Flanked by Secretary of State Hillary Clinton in the White House Rose Garden, Obama hailed the Libyan security personnel who joined their U.S. counterparts in fighting to protect the consulate.

He stressed that the United States would continue working with the Libyan government to stabilize the country, which has been plagued by a stream of violent incidents by rebel militias and Islamic extremist groups that refused to disband after Gadhafi’s October 2011 ouster.

"This attack will not break the bonds between the United States and Libya," said Obama.

As of August 2012, the U.S. government had provided more than $200 million in assistance to Libya since the start of the uprising in 2011. The aid includes $89 million in humanitarian assistance, $40 million for rounding up weapons, and $25 million in nonlethal U.S. military supplies.

Obama praised Stevens for the "characteristic skill, courage and resolve" with which he worked "to build a new Libya" as the U.S. envoy to the anti-Gadhafi rebels and then the U.S. ambassador.

"It is especially tragic that Chris Stevens died in Benghazi, because it is a city that he helped to save," Obama said.

Stevens was among the U.S. officials who advocated the U.S.-backed NATO intervention in the Libyan civil war as Gadhafi’s forces were moving to overrun Benghazi, the headquarters of the rebellion.

Speaking at the State Department before going to the White House, Clinton said, "I ask 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, how confounding the world can be.”