[The indictment] said that in the days before the attack, he voiced “concern and opposition to the presence of an American facility in Benghazi,” the government said.

On the night of Sept. 11, 2012, a group of at least 20 men armed with machine guns, handguns and rocket-propelled grenades gathered outside the United States Mission in Benghazi and “aggressively breached” its gate, according to the document.

The men went on to set fire to the United States Mission. It was that fire that killed Mr. Stevens and a State Department employee. A little later, Mr. Abu Khattala “entered the compound and supervised the exploitation of material from the scene by numerous men, many of whom were armed.”

He then went to one of his militia’s camps, where many of its members had gathered and prepared a second attack, on another American outpost. Fearing that the United States was going to retaliate after the attacks, he tried to obtain weapons in the following days.