Members of various anti-American jihadi groups based in and around Benghazi, according to CIA intelligence. One possible ringleader is Ahmed Abu Khattala, a deeply violent militia leader with ties to more radical Benghazi organizations. Khattala denies culpability for the attack, he’s been indicted by a federal jury on charges of helping plan and organize the attack. Since no one group is claiming responsibility, it’s hard to know for sure what the motivation for the attack was beyond a general jihadi objective of killing Americans.
There’s no real evidence as yet that any of these groups planned the attack in advance — in other words, it appears to have been largely spontaneous. In tactical terms, the initial attack on the mission wasn’t that complicated — a bunch of guys with easy access to light weapons hit a poorly guarded American target.
The US intelligence community now believes the attack was primarily an alliance of opportunity between some militants, rather than a pre-planned offensive. “The nature of the attacks suggested they did not involve significant pre-planning,” then-acting CIA Director Michael Morell wrote to Sen. Dianne Feinstein on January 4th, 2013, summing up the consensus among US intel agencies.