this article ignores the possibility that the united states - which has the largest stockpiles of chemical weapons on earth - may have smuggled in the sarin through it's contacts with the rebels, which is the theory i'm operating under.

the united states routinely ships weapons to these rebels, and nobody has the authority to inspect these shipments.

http://www.chicagotribune.com/news/nationworld/ct-syria-chemical-weapons-20170408-story.html