'Credible' That ISIS Used Chemical Agent Against Kurdish Fighters

"Blistering Agent" Was Used Earlier This Week

A U.S. official said the attack occurred several days ago and involved a small amount of blistering agent that affected a small number of Kurdish fighters.

It is unclear how ISIS may have obtained the undetermined chemical agent.

Mustard gas is the most well-known blistering agent, but officials cautioned that no determination had been made as to whether mustard gas was used.

They noted that there are many kinds of blistering agents that cause similar skin reactions to those seen in the attack.

The U.S. official said this latest incident is believed to be credible based on the attack a few days ago, though no final determination has been made.