The Organisation for the Prohibition of Chemical Weaponshad's Fact-Finding Mission established the use of Sarin Gas in a report (PDF) released on 2017, June 29.
THE HAGUE, Netherlands — 30 June 2017 — In a report released by the Organisation for the Prohibition of Chemical Weapons (OPCW), the OPCW Fact-Finding Mission (FFM) confirmed that people were exposed to sarin, a chemical weapon, on 4 April 2017 in the Khan Shaykhun area, Idlib Province in the Syrian Arab Republic.
OPCW and the United Nations have formed a Joint Investigation Mechanism to investigate allegations of chemical attacks in Syria, and in particular the case of the attacks on Khan Sheikhoun. It has been mandated to establish the responsibilities for this event.
The report (extracts / PDF), released on 26 october 2017, concluded that a Syrian Arab Air Force aircraft was responsible for dropping the sarin during the attack.
The report documents why (one of) the explanation offered by Russia (a bomb strike on a sarin stocking site) is implausible : the nerve agent would then have been burnt and buried and not released in such huge quantities in the air. It also studies the hypothesis of an artisanal ground bomb releasing sarin gas, and dismisses it.
It concludes that a Syrian Soukhoï-22 attacked at 6:45 am, dropping first three conventional bombs and then one chemical bomb.
Evidence collected includes satellite images, interviews with witnesses and victims, communications from fighters, photos and videos including some images of bombs remnants, and analysis of samples.