This image made from video released by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows a medical worker administering oxygen through respirators following an alleged poison gas attack in Douma. The Syrian government denied the allegations, which could not be independently verified. (Syrian Civil Defense White Helmets via AP)

This image made from video released by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows a medical worker administering oxygen through respirators following an alleged poison gas attack in Douma. The Syrian government denied the allegations, which could not be independently verified. (Syrian Civil Defense White Helmets via AP)