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.