CAIRO — After a month of conflicting statements and partisan criticism, the circumstances surrounding the attack that killed Ambassador J. Christopher Stevens in Benghazi, Libya, on Sept. 11 have become clouded in ambiguities and questions: Did the attack grow out of anger against an American-made video mocking the Prophet Muhammad, or was it waged by an affiliate of Al Qaeda out to mark the 11th anniversary of its attack on United States soil?

To Libyans who witnessed the assault and know the attackers, there is little doubt what occurred: a well-known group of local Islamist militants struck the United States Mission without any warning or protest, and they did it in retaliation for the video. That is what the fighters said at the time, speaking emotionally of their anger at the video without mentioning Al Qaeda, Osama bin Laden or the terrorist strikes of 11 years earlier. And it is an explanation that tracks with their history as members of a local militant group determined to protect Libya from Western influence.

“It was the Ansar al-Shariah people,” said Mohamed Bishari, 20, a neighbor of the compound who watched the assault and described the brigade he saw leading the attack. “There was no protest or anything of that sort.”

United States intelligence agencies have reserved final judgment pending a full investigation, leaving open the possibility that anger at the video might have provided an opportunity for militants who already harbored anti-American feelings. But so far the intelligence assessments appear to square largely with local accounts. Whether the attackers are labeled “Al Qaeda cells” or “aligned with Al Qaeda,” as Republicans have suggested, depends on whether that label can be used as a generic term for a broad spectrum of Islamist militants, encompassing groups like Ansar al-Shariah whose goals were primarily local, as well as those who aspire to join a broader jihad against the West.