BENGHAZI, Libya -- The U.S. ambassador to Libya and three other American officials died in a coordinated assault on the U.S. consulate by gunmen firing assault rifles and rocket-propelled grenades and carrying the black flag of an Islamic extremist group, the property's landlord said Wednesday.

Standing outside the fire-gutted compound, Mohammad al Bishari denied the attack began as a protest against an amateurish U.S.-made video mocking the Prophet Muhammad, the founder of the Islamic faith.

"They attacked right away," Bishari said.

Bishari said he believed that U.S. Ambassador Christopher Stevens and State Department computer specialist Sean Smith died from inhaling smoke spewed from a fire set by the assailants. U.S. officials corroborated much of Bishari's account and said that two other American officials were killed by gunfire at a consulate annex.

In the wake of the deaths of Stevens -- the first U.S. ambassador killed in more than 30 years -- and the other Americans, U.S. diplomatic and military facilities around the world tightened security and urged U.S. citizens to take precautions to avoid being caught up in further violence.

The U.S. Embassy in Sudan's capital, Khartoum, evacuated it staff; the consulate in Casablanca, Morocco, closed; and U.S.-led forces in war-wracked Afghanistan were placed on alert.

All but a skeleton crew of U.S. personnel were flown to Europe from Libya, protected by 50 Marines who will remain in the country while the security situation is assessed. American non-governmental organizations also began evacuating their staff from Tripoli.

At the U.S. Embassy in Cairo, where demonstrators ripped and burned the American flag on Tuesday, crowds gathered once again outside the embassy building, though there was no repeat of Tuesday's mayhem.

In Washington, President Barack Obama vowed to hunt down the gunmen who staged the "outrageous and shocking attack" on the Benghazi consulate.

"Make no mistake, we will work with the Libyan government to bring to justice the killers who attacked our people," Obama said in a brief remarks at the White House. He condemned the attack as "outrageous and shocking."

Bishari said the attack began with assailants carrying assault rifles, rocket-propelled grenades and the black flag of Ansar al Sharia -- The Partisans of Sharia -- moving from two directions against the compound, which is made up of a main building and a number of smaller ones.

"Whatever they didn't loot, they burned," he said.

Stevens, who was visiting from the capital, Tripoli, three other Americans and six Libyan security guards were inside the main building, Bishari said. The Libyan security guards managed to carry Stevens, apparently overcome by smoke, out of the building, place him in a car and drive out of compound's back gate to a hospital, where he died, Bishari said.

Stevens was the first U.S. ambassador killed since Feb. 14, 1979, when the U.S. envoy to Afghanistan, Adolph Dubs, was kidnapped and shot dead by armed militants.

In Washington, two administration officials, briefing reporters on the condition of anonymity, corroborated much of Bishari's description of what happened.

They said the assault began around 10 p.m. local time when gunmen began firing into the compound. Fifteen minutes later, the attackers gained access to the compound and set the main building aflame. Stevens, Smith and an unidentified U.S. security officer were inside. As dense smoke filled the building, the security officer was separated from Stevens and Smith and managed to get outside, the administration officials said.

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-Zawahri 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 Moammar 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.