"Alive, Alive! God is great," the crowd erupts, while someone calls to bring Mr. Stevens to a car.

Mr. Stevens was taken to a hospital, where a doctor tried to revive him, but said he was all but dead on arrival.

The full identity and motivation of the attackers remains a matter of dispute. Considerable suspicion has fallen on a local Benghazi militia, Ansar al-Sharia, known for its intensely conservative and anti-democratic Islamist politics. Witnesses saw the group's insignia on trucks at the scene, and attackers acknowledged they were members. Fighters and others present at the attack said the motive was anger at a video produced in the United States that denigrates the Prophet Muhammad.

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On Sunday, Mohamed Yussef Magariaf, president of Libya's newly elected national congress, said in interviews with American news media that he believed people affiliated with or sympathetic to Al Qaeda played a role in the assault, although he did not seem to rule out that the attackers might have been ideological allies of Al Qaeda without specific collaboration. The regional Qaeda affiliate, Al Qaeda in the Islamic Maghreb, is active near Libya but has focused primarily on attacking local governments.

Speaking on CBS's "Face the Nation," Mr. Magariaf said that Libya has arrested as many as 50 people over the assault. At least a few, he said, had come from outside Libya, possibly Algeria or Mali. And he also said that he believed the non-Libyans had been involved in planning the attack in the months since they entered the country, and that it was meant to coincide with the anniversary of the Sept. 11 attacks.

Referring to "ugly deeds, criminal deeds," Mr. Magariaf insisted that the attacks "do not resemble any way, in any sense, the aspirations, the feelings of Libyans towards the United States and its citizens," emphasizing the role of "foreigners."

Appearing on the same program, Susan Rice, the United States ambassador to the United Nations, said the attacks began "spontaneously in Benghazi as a reaction to what had transpired some hours earlier in Cairo."

"But soon after that spontaneous protest began outside of our consulate in Benghazi, we believe that it looks like extremist elements, individuals, joined in that effort with heavy weapons of the sort that are, unfortunately, readily now available in Libya post-revolution," Ms. Rice said. "And that it spun from there into something much, much more violent."

The United States did not believe the attack was preplanned or premeditated, Ms. Rice said, adding that whether the extremists "were Al Qaeda affiliates, whether they were Libyan-based extremists or Al Qaeda itself I think is one of the things we'll have to determine."