In this case, a central figure in the attack was an eccentric, malcontent militia leader, Ahmed Abu Khattala, according to numerous Libyans present at the time. American officials briefed on the American criminal investigation into the killings call him a prime suspect. Mr. Abu Khattala declared openly and often that he placed the United States not far behind Colonel Qaddafi on his list of infidel enemies. But he had no known affiliations with terrorist groups, and he had escaped scrutiny from the 20-person C.I.A. station in Benghazi that was set up to monitor the local situation.
Mr. Abu Khattala, who denies participating in the attack, was firmly embedded in the network of Benghazi militias before and afterward. Many other Islamist leaders consider him an erratic extremist. But he was never more than a step removed from the most influential commanders who dominated Benghazi and who befriended the Americans. They were his neighbors, his fellow inmates and his comrades on the front lines in the fight against Colonel Qaddafi.
To this day, some militia leaders offer alibis for Mr. Abu Khattala. All resist quiet American pressure to turn him over to face prosecution. Last spring, one of Libya’s most influential militia leaders sought to make him a kind of local judge.
Fifteen months after Mr. Stevens’s death, the question of responsibility remains a searing issue in Washington, framed by two contradictory story lines.
One has it that the video, which was posted on YouTube, inspired spontaneous street protests that got out of hand. This version, based on early intelligence reports, was initially offered publicly by Susan E. Rice, who is now Mr. Obama’s national security adviser.
The other, favored by Republicans, holds that Mr. Stevens died in a carefully planned assault by Al Qaeda to mark the anniversary of its strike on the United States 11 years before. Republicans have accused the Obama administration of covering up evidence of Al Qaeda’s role to avoid undermining the president’s claim that the group has been decimated, in part because of the raid that killed Osama bin Laden.
The investigation by The Times shows that the reality in Benghazi was different, and murkier, than either of those story lines suggests. Benghazi was not infiltrated by Al Qaeda, but nonetheless contained grave local threats to American interests. The attack does not appear to have been meticulously planned, but neither was it spontaneous or without warning signs.
