The claim of the 'chemical attack' was made shortly after U.S. President Trump had announced that he wanted U.S. troops to leave Syria. It was designed to "pull him back in" which it indeed did. In an illegal 'retaliation' the U.S., Britain and France launched a number of cruise missiles against Syria. Most of them failed to reach their targets.
Moon of Alabama published a number of pieces on the issue which are listed below.
It seemed obvious from the very first claims of the 'gas attack' that it did not happen at all. The Syrian government had no motive to use any chemical weapon or an irritant like chlorine in Douma. It had already won the battle. The incident was obviously staged, like others before it, to drag the U.S. into a new attack on Syria.
Of special interest on the incident scene were two gas cylinders which were photo- and video-graphed near to where the dead bodies were found. It was claimed that the cylinders were dropped from Syrian army helicopters and crashed through concrete roofs. One cylinder allegedly 'bumped' after completely penetrating the roof and came to rest on a bed. The other cylinder allegedly broke a roof open and came to rest on a balcony.