Chris Stevens, an American who risked his life to help Libyans overthrow dictator Moammar Gadhafi, was killed overnight in the former rebel capital of Benghazi — a city he helped save, making it an especially tragic place for him to die, President Obama said Wednesday.

An Arabic speaker who loved Libya and understood it deeply, Stevens died along with three other Americans when an angry mob stormed the U.S. Consulate in Benghazi. He was the first U.S. ambassador to be killed in the line of duty since 1979.

The consulate was one of several American diplomatic missions in the Middle East to face protests Tuesday after the release online of a film mocking Islam and depicting the Muslim prophet Mohammed as a child molester, womanizer and ruthless killer.

But U.S. sources told CNN on Wednesday that the Benghazi attack was planned in advance, and the attackers used the protest outside the consulate as a diversion. The sources could not say whether the attackers instigated the protest or merely took advantage of it, and they say they don’t believe Stevens was specifically targeted.

A senior U.S. official familiar with the details of the attack said a grenade set the building ablaze, leaving the Americans facing both a fire inside and attackers outside. Stevens and the others who died were separated from the rest of the staff while trying to escape to the roof of the building and succumbed to smoke inhalation, the senior official said.

The official said there were several “valiant but unsuccessful” attempts to get back into the building and rescue them.

Meanwhile, a London think tank with strong ties to Libya speculated Wednesday that Stevens was actually the victim of a targeted al Qaeda revenge attack. The assault “came to avenge the death of Abu Yaya al-Libi, al Qaeda’s second in command killed a few months ago,” the think tank Quilliam said Wednesday.

It was “the work of roughly 20 militants, prepared for a military assault,” the think tank said, noting that rocket-propelled grenade launchers do not normally appear at peaceful protests, and that there were no other protests against the film elsewhere in Libya.

In Washington, President Barack Obama said the violence was completely unwarranted and vowed that “justice will be done.”

“Since our founding, the United States has been a nation that respects all faiths. We reject all efforts to denigrate the religious beliefs of others,” he said. “But there is absolutely no justification for this type of senseless violence — none.”

– CNN.com