It's been burbling up from the conservative media for nearly six months, starting with Fox News. Last year, the network's reporter Catherine Herridge reported on a ship that had arrived to Turkey from Libya laden with weapons. Ordnance left unsecured after the fall of Gaddafi was being taken to Syria to overthrow another dictator.
This isn't in much dispute. The dispute, and the theory, is that the weapons used to kill Americans in Benghazi were made available by bungling American gun-runners. That's the theory floated by Roger Simon, who talks to two "Benghazi whistleblowers" (multiplying like rabbits now).
[Chris] Stevens’ mission in Benghazi, they will say, was to buy back Stinger missiles from al-Qaeda groups issued to them by the State Department, not by the CIA. Such a mission would usually be a CIA effort, but the intelligence agency had opposed the idea because of the high risk involved in arming “insurgents” with powerful weapons that endanger civilian aircraft.
It's a nearly perfect scandal—Fast and Furious plus Benghazi, a sort of Neapolitan sundae of outrage and disgrace. If the anonymous accusers are wrong, we have plenty of other ways to explain the loose weapons in Benghazi and the transfer to Syria. And making it possible for the stray weapons to get to Syria is the sort of thing both parties in Congress largely favor. But the darkest version of the theory is gaining ground on the right.