In 2017, Postol criticized the unclassified intelligence assessment released by the Trump administration blaming the air forces of Syrian President Bashar al-Assad for the April Khan Shaykhun chemical attack. Postol analysed the photographic evidence and concluded that the chemical attack was not an air raid, but was conducted from the ground using most probably an emptied 122mm artillery rocket tube, which is normally used as munition of a multiple rocket launcher, filling it with a chemical agent and detonating it by an explosive charge laid on top of it. On 18 April, Postol published his findings that the crater present in the photographs could not have been the source of the necessary sarin smoke plume, as persons were seen in the video material live at the site, without appropriate protection gear. A sarin smoke plume would have killed them even hours after a sarin gas grenade explosion. On 21 April, he corrected this view in one aspect: "In my earlier report released on April 18, 2017 I misinterpreted the wind-direction convention which resulted in my estimates of plume directions being exactly 180° off in direction", which was however irrelevant for his main statement, that no sarin could have been emitted at the crater-site. Later in April, Postol wrote that the "French Intelligence Report of April 26, 2017 contradicted the White House Intelligence Report of 11 April 2017". The following day he revised his view, saying he had confused the date and location for a different chemical attack four years earlier. Postol stated that none of the forensic evidence in "The New York Times" video and a follow-on "Times" news article on the Khan Shaykhun chemical attack supported the conclusions reported by "The New York Times".