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Chris Stevens, the U.S. ambassador to Libya killed along with three others in a rocket attack on the American Consulate in Benghazi was "a courageous and exemplary representative of the United States," President Barack Obama said in his initial statement Wednesday condemning the attack.

Initial reports said the slain embassy staffers—who also include foreign service information management officer Sean Smith—were trying to flee the consulate building that was under assault by protesters apparently angry over a film they say insults Prophet Muhammad.

However, U.S. officials told The New York Times and CNN on Wednesday that the attack may have been planned by a group that "had either been waiting for an opportunity to exploit like the protests over the video or perhaps even generated the protests as a cover for their attack."

[Also read: President Obama's statement on attack in Benghazi]

According to the Associated Press, a Libyan doctor who treated Stevens said the diplomat died of severe asphyxiation from smoke inhalation and that he tried for 90 minutes to revive him.

Stevens, 52, is the sixth ambassador to die in an attack in U.S. history, according to CBS News, and first since 1988, when Arnold Raphel, U.S. ambassador to Pakistan, was killed in a plane crash there.

"Throughout the Libyan revolution, [Stevens] selflessly served our country and the Libyan people at our mission in Benghazi," Obama said. "As ambassador in Tripoli, he has supported Libya's transition to democracy. His legacy will endure wherever human beings reach for liberty and justice. I am profoundly grateful for his service to my administration, and deeply saddened by this loss."

Stevens, a California native and U.C.-Berkeley grad, was a 21-year veteran of foreign service, the White House said.

"I had the privilege of swearing in Chris for his post in Libya only a few months ago," Secretary of State Hillary Clinton said in a separate statement. "He spoke eloquently about his passion for service, for diplomacy and for the Libyan people. This assignment was only the latest in his more than two decades of dedication to advancing closer ties with the people of the Middle East and North Africa.

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"As the conflict in Libya unfolded, Chris was one of the first Americans on the ground in Benghazi," Clinton continued. "He risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation. He spent every day since helping to finish the work that he started. Chris was committed to advancing America's values and interests, even when that meant putting himself in danger."

In response to the attack, the United States is "deploying elite Marine counterterrorism teams to Libya," Foreign Policy reports. The Pentagon is sending Fleet Anti-Terrorism Teams, or FAST teams, a U.S. defense official told the magazine. According to Reuters, the United States has evacuated all personnel from Benghazi to Tripoli.

"It's especially tragic because Chris Stevens died in Benghazi because it is a city he fought to save," Obama said later Wednesday morning in hastily arranged public remarks in the Rose Garden of the White House.

Speaking at an impromptu press conference in Jacksonville, Fla., on Wednesday, Mitt Romney condemned Tuesday's attacks as "disgusting" and "outrageous," but he also attacked the Obama administration for standing by a statement issued by the U.S. Embassy in Cairo that he claimed was an "apology" for American values.

[Related: Romney attacks Obama for sending 'mixed signals' on Middle East violence]

Late Tuesday, Romney issued a statement saying it was "disgraceful that the Obama administration's first response was not to condemn attacks on our diplomatic missions but to sympathize with those who waged the attacks." On Wednesday, he stood by his criticism of the White House.

Meanwhile, a photograph purportedly showing Stevens' body was published online by several news outlets, including the New York Times, sparking an outcry from readers and a debate among editors.

Here's Stevens' bio from the U.S. Embassy website:

Ambassador Chris Stevens considers himself fortunate to participate in this incredible period of change and hope for Libya. As the President's representative, his job is to develop a strong, mutually beneficial relationship between the United States and Libya. Ambassador Stevens was the American representative to the Transitional National Council in Benghazi during the revolution. When he's not meeting with government officials or foreign diplomats, you can find Ambassador Stevens meeting with Libyan academics, business people, and civil society activists, exploring Libya's rich archaeological sites, and enjoying Libya's varied cuisine. After several diplomatic assignments in the Middle East and North Africa, Ambassador Stevens understands and speaks Arabic and French. He likes the Facebook page of the U.S. Embassy in Tripoli and hopes you will, too.

"I had the honor to serve as the U.S. envoy to the Libyan opposition during the revolution," Stevens said in May in a video introducing himself to the Libyan people as the new U.S. ambassador there. "And I was thrilled to watch the Libyan people stand up and demand their rights.

[Slideshow: Gunmen storm U.S. consulate in Libya]

"Growing up in California I didn't know much about the Arab world," Stevens continued. "I traveled to North Africa as a Peace Corps volunteer, worked as an English teacher in a town in the high Atlas mountains in Morocco for two years and quickly grew to love this part of the world.

"We know Libya is still recovering from an intense period of conflict," he added. "There are many courageous Libyans who wear the scars of that battle."