One of the most damning details from the United Nations investigation has to do with where the the sarin-filled artillery shells appear to have been fired from. They appeared to sail in from the northwest – from a part of Damascus that just happens to be tightly controlled by Syrian regime forces and to contain a large Republican Guard base. Human Rights Watch, which conducted its own investigation that concluded that the Assad regime was likely responsible, actually put together this map of the attacks based on the U.N. data. It seems to point pretty squarely to the Assad regime:
As you can see, the chemical weapons appear to have been launched not just from a regime-controlled area but from within a Republican Guard military base. That's tough to argue with. Here, from Human Rights Watch's own data, is a wider view of the same area, showing broader regime- and rebel-controlled areas:
If the chemical weapons had been fired by rebels, presumably they would have come from the rebel-held southeast rather than the regime-controlled northwest. New York Times conflict reporter C.J. Chivers, writing on his personal blog, concluded that the theory that the rebels may have launched the chemical weapons "essentially evaporates" with this new evidence. "Viewed through a common-sense understanding of the limits and conditions of the battlefield, the rebels could not have done this," he writes. "Claims of rebel culpability are now specious; technically and tactically implausible, they are too outlandish for even a sci-fi script."
This is not conclusive proof that the Assad regime was behind the chemical attack – we don't have a video of Assad holding up an Aug. 21 newspaper and a copy of his birth certificate while he orders Republican Guard troops to blanket a civilian neighborhood with sarin gas. And, to be fair, it is still possible that the attack may have been fired by a rogue contingent of troops or ordered by a freelancing general. But this map makes the official Russian claim — that rebels launched sarin gas at Syrian civilians to provoke a Western response — look a lot harder to defend.