What he did in the period just before the attack has remained unclear. But Mr. Abu Khattala told other Libyans in private conversations during the night of the attack that he was moved to attack the diplomatic mission to take revenge for an insult to Islam in an American-made online video.

An earlier demonstration venting anger over the video outside the American Embassy in Cairo had culminated in a breach of its walls, and it dominated Arab news coverage. Mr. Abu Khattala told both fellow Islamist fighters and others that the attack in Benghazi was retaliation for the same insulting video, according to people who heard him.

In an interview days after the attack, he pointedly declined to say whether he believed an offense such as the anti-Islamic video might indeed warrant the destruction of the diplomatic mission or the killing of the ambassador. “From a religious point of view, it is hard to say whether it is good or bad,” he said.