The descriptions of victims arriving at clinics near Damascus on Wednesday point to possible signs of poisoning by a nerve agent, the most deadly of the seven types of chemical weapons recognized by experts.

Nerve agents can kill quickly, within 10 minutes. Initial symptoms include salivation, constriction of the pupils and a feeling of tightness in the chest. At high doses, muscles clench, twitch and spasm.

"Your muscles tense up, but they can't release," said Charles Blair, a senior fellow on state and nonstate threats for the Federation of American Scientists. "There's muscle twitching. Then, as the muscle twitching gets more and more spasmodic, mucus comes out of the nose and mouth and you basically go into convulsions on the ground. People don't survive this."