TRIPOLI, Libya — The American ambassador to Libya and three other Americans were killed when a mob of protesters and gunmen overwhelmed the U.S. Consulate in Benghazi, setting fire to it in outrage over a film that ridicules Islam's Prophet Muhammad. Libya's new president apologized Wednesday for the attack, which underlined the lawlessness plaguing a region trying to recover from months of upheaval.

Ambassador Chris Stevens, 52, died as he and a group of embassy employees went to the consulate to try to evacuate staff as a crowd of hundreds attacked the consulate Tuesday evening, many of them firing machine-guns and rocket-propelled grenades.

By the end of the assault, much of the building was burned out and trashed. Stevens was the first U.S. ambassador to be killed in the line of duty since 1979.

A Libyan doctor who treated Stevens said he died of severe asphyxiation, apparently from smoke. In a sign of the chaos of during the attack, Stevens was brought alone by Libyans to the Benghazi Medical Center with no other Americans, and no one at the facility knew who he was, the doctor, Ziad Abu Zeid, told The Associated Press.

Stevens was practically dead when he arrived close to 1 a.m. on Wednesday, but "we tried to revive him for an hour and a half but with no success," Abu Zeid said. The ambassador had bleeding in his stomach because of the asphyxiation but no other injuries, he said.

President Barack Obama ordered increased security to protect American diplomatic personnel around world. Hours before the Benghazi attack, Egyptians angry over the film protested at the U.S. Embassy in Cairo, climbing its walls and tearing down an American flag, which they replaced briefly with a black, Islamist flag.

"I strongly condemn the outrageous attack on our diplomatic facility in Benghazi," Obama said, adding the four Americans "exemplified America's commitment to freedom, justice, and partnership with nations and people around the globe."

Libya's interim president, Mohammed el-Megarif, apologized to the United States for the attack, which he described as "cowardly." Speaking to reporters, he offered his condolences on the death of the four Americans and vowed to bring the culprits to justice and maintain his country's close relations with the United States.

The three Americans killed with Stevens were security guards, he said.

"We extend our apology to America, the American people and the whole world," el-Megarif said.

The spark for the protests in Libya and Egypt was an obscure movie made in the United States by a California filmmaker who calls Islam a "cancer." Video excerpts posted on YouTube depict Muhammad as a fraud, a womanizer and a madman in an overtly ridiculing way, showing him having sex and calling for massacres.

But the brazen assaults — the first on U.S. diplomatic facilities in either country — underscored the lawlessness that has taken hold in Libya and Egypt after revolutions ousted their autocratic secular regimes and upended the tightly controlled police state in both countries.

Islamists, who were long repressed under the previous regimes, have emerged as a powerful force and made up the bulk of the protests in both countries.

Moreover, security in both countries has broken down. Egypt's police, a onetime hated force blamed for massive human rights abuses, have yet to fully take back the streets after Hosni Mubarak's ouster in February 2011.

On Tuesday in Cairo, riot police stood by the embassy's walls but continued to allow protesters to climb them for several hours. The protesters, however, appeared to intentionally stick to certain limits: A few entered the embassy grounds to remove the flags and come back, but otherwise the chanting youth stayed on top of the walls without storming the compound or damaging property.

The uproar over the film also poses a new test for Egypt's new Islamist president, Mohammed Morsi, who has yet to condemn the riot outside the U.S. Embassy in Cairo or say anything about the offending film. The protest was by mostly ultraconservative Islamists.

In Libya, central government control is weak, arms are ubiquitous and militias are pervasive. The consulate in Benghazi, Libya's second largest city, is a one-story villa in a large garden located in an upscale neighborhood. By the end of Tuesday night's attack, much of the building was black and smoldering. Libyans wandered freely around the burned-out building, taking photos of rooms where furniture was covered in soot and overturned.

The violence raised worries that further protests could break out around the Muslim world as knowledge of the anti-Islam movie spread. So far, however, the only sign of unrest on Wednesday was a protest by dozens of Gazans in Gaza City. Some of the protesters carried swords, axes and black flags, chanting, "Shame on everyone who insults the prophet." The rally was organized by supporters of the Popular Resistance Committees, a militant group aligned with the ruling Hamas movement.

Afghanistan's government sought to avert an outbreak of protests. President Hamid Karzai condemned the movie, which he describes as "inhuman and insulting." Authorities also temporarily shut down access to YouTube, the video-sharing site where excerpts of the movie were posted, said Aimal Marjan, general director of Information Technology at the Ministry of Communications.

Ultraconservative Islamists also were suspected of being behind the Benghazi attack. Advocating a strict interpretation of Islam, they have bulldozed Sufi shrines and mosques that house tombs in the Libyan capital, Tripoli, and other cities, including ancient sites dating back to 5,000 years ago.

Heavily armed, ultraconservative groups like Ansar al-Shariah, or Supporters of Shariah, have claimed responsibility for the attacks on the shrines, declaring Sufi practices as "heretical."

Libya has been also hit by a series of recent attacks that served as evidence of the deep and persistent security vacuum in the country after the fall of Moammar Gadhafi's regime, which was ousted by rebels backed by a NATO air campaign. Many Libyans believe that unrest in their country is in part the work of Gadhafi's loyalists who want to undermine efforts to rebuild the country after last year's ruinous civil war.

Stevens was a career diplomat who 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.

Before Tuesday, five U.S. ambassadors had been killed in the line of duty, the last being Adolph Dubs in Afghanistan in 1979, according to the State Department historian's office.

The two-hour movie that sparked the protests, titled "Innocence of Muslims," came to attention in Egypt after its trailer was dubbed into Arabic and posted on YouTube.

Sam Bacile, a 56-year-old California real estate developer, said he wrote, produced and directed the movie. Bacile told The Associated Press he was an Israeli Jew and an American citizen. But Israeli officials said Wednesday they had not heard of Bacile and there was no record of him being a citizen. They spoke on condition of anonymity because they are not permitted to share personal information with the media.

Separately, the film was being promoted by an extreme anti-Muslim Egyptian Christian campaigner in the United States.

Bacile said he had not anticipated such a furious reaction. Speaking by phone from an undisclosed location, Bacile, who went into hiding Tuesday, remained defiant. He said he believes the movie will expose Islam's flaws to the world.

"Islam is a cancer, period," he repeatedly said in a solemn, accented tone.

Israel, however, sought to distance itself from Bacile.

"It's obvious we'll have to be vigilant. Anything he did or said has nothing to do whatsoever with Israel. He may claim what he wants. This was not done with or for or through Israel." Foreign Ministry spokesman Yigal Palmor said on Wednesday.