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S/2017/904 20/33 17-18978 Annex II Khan Shaykhun I. Findings of the Organisation for the Prohibition of Chemical Weapons Fact-Finding Mission in the Syrian Arab Republic 1. On the basis of its analysis of biomedical specimens, interviews and supplementary material submitted during the interview process, as well as analysis of environmental samples, the Organisation for the Prohibition of Chemical Weapons (OPCW) Fact-Finding Mission in the Syrian Arab Republic found that a large number of people, some of whom had died, had been exposed to sarin or a sarin-like substance at Khan Shaykhun on 4 April 2017. While it had received limited information on the dispersal mechanism and therefore had been unable to draw firm conclusions on that specific matter, it considered that the release that had caused the exposure was likely to have been initiated in the crater in the road, located close to the silos in the northern part of the town. It concluded that, on the basis of such a release, the only determination that could be made was that sarin had been used as a weapon. II. Investigation by the Mechanism 2. Upon receiving the final report of the Fact-Finding Mission on the incident of 4 April 2017 at Khan Shaykhun, the Mechanism conducted an in-depth investigation into the incident. The Leadership Panel defined the scope of the investigation by adopting an investigation plan that outlined eight possible scenarios, including those put forward by Member States, regarding how sarin had been released in Khan Shaykhun. While the Mechanism sought to collect and analyse as much information as possible in connection with each scenario, by the end of the investigation most of the information obtained supported three scenarios: (a) sarin had been delivered through an aerial bomb that had been dropped by an aircraft; (b) sarin had been released from the ground as part of a staged attack; or (c) there had been an air strike by the Syrian Arab Air Force against a storage facility containing toxic chemicals, which had resulted in the dispersion of a toxic cloud. 3. The first scenario is based on reports that aircraft were either seen or heard dropping bombs over Khan Shaykhun early in the morning on 4 April 2017. Sarin is believed to have emanated from a point of impact on a road (hereinafter referred to as “the crater”) caused by an aerial bomb, located close to silos in the northern part of Khan Shaykhun. 4. The second scenario is based on a report that sarin was released from the same crater as that in the first scenario, caused by an explosive charge placed on the ground containing sarin, so as to stage an attack for which the Government of the Syrian Arab Republic would be blamed. 5. The third scenario involves a reported strike by the Syrian Arab Air Force on an ammunition depot on the eastern outskirts of Khan Shaykhun where workshops had been producing chemical warfare munitions. In a public statement, the Government of the Syrian Arab Republic stated that the Syrian Arab Air Force had only conducted an attack in Khan Shaykhun at around noon on 4 April 2017. An alternative third scenario is that a house in Khan Shaykhun that had been taken over by a non-State armed group and used for the storage of toxic chemicals was bombed on 4 April 2017, thus releasing toxic chemicals