Washington said it believed the deaths were caused by sarin nerve gas dropped by Syrian aircraft. But Moscow offered an alternative explanation that would shield Assad: that the poison gas belonged to rebels and had leaked from an insurgent weapons depot hit by Syrian bombs.
A previous assertion that rebel elements used sarin is Seymour Hersh’s piece, (LRB) Whose Sarin?, about the August 2013 incident near Damascus. It shows the power of good writing. But it was completely refuted by the U.N. trajectory study cited by the (BBC) in Syria chemical attack: What we know. Quoting from (pdf, undocs.org) “Identical letters dated 13 December 2013 from the Secretary-General addressed to the President of the General Assembly and the President of the Security Council”, document # A/68/663–S/2013/735,
8. In a letter dated 14 June 2013, the Permanent Representative of the United States of America to the United Nations reported to the Secretary-General, inter alia, its updated assessment alleging that the Government of the Syrian Arab Republic had used the chemical warfare agent Sarin in an attack on the Aleppo suburb of Khan Al Asal on 19 March 2013.
But the difficulties are simplified for production of binary sarin, which is stored in a bomb or projectile as two chemicals that mix when the bomb is released or the projectile is fired. It is possible that ISIS could operate a binary production facility. What would happen if the facility were hit by a bomb?
Mixing of the two precursors must be done thoroughly and quickly, or the production of sarin is low. A hit on a factory would cause most of the contents to separate, rather than combine. Dispersion would be slow and wind driven. Nevertheless, if tons of a sarin precursor chemical, methylphosphonic acid dichloride, was stored on site and dispersed, fatalities could have resulted.
But most significantly, (NY Times) Worst Chemical Attack in Years in Syria; U.S. Blames Assad quotes witness Mariam Abu Khalil, who was extremely close to the impact:
“…she saw an aircraft drop a bomb on a one-story building a few dozen yards away. In a telephone interview Tuesday night, she described an explosion like a yellow mushroom cloud that stung her eyes. “It was like a winter fog,” she said.”
Her recounting does not describe a blast effect, the shock to the body from proximity to an explosion. The absence is specific to a chemical munition. This, and prior use of sarin by the Assad regime, effectively refute the Russian argument.