This article includes graphic scenes of injury and death.

Facebook/Qasioun Agency, via Storyful

The videos, shared by activists and residents, showed children and older people gasping and struggling to breathe. Here, a child is treated in the town of Khan Sheikhoun.

Medical workers and witnesses said that chlorine attacks, which have become frequent in northern Syria, usually kill just a few people, often those trapped in an enclosed space, and the gas dissipates quickly. This time, they saw people collapsing outdoors, in much larger numbers, and dozens of people died.

Edlib Media Center

Some lay motionless in the mud as rescue workers ripped off victims’ clothes and hosed them down.

EMC and Syria Civil Defense

Witnesses said the gas was delivered by a government airstrike. The attacks raised the possibility that the Syrian government used a banned nerve agent, like sarin, after it agreed in 2013 to eliminate its chemical weapons program.

In this attack, the symptoms included pupils reduced to pencil-point dots, a marker associated with exposure to nerve agents. One doctor posted evidence of what he was seeing.

YouTube/Shajul Islam

Other victims were seen foaming at this mouth, another possible symptom of nerve-agent exposure.

Facebook/Syrian Civil Defense