Dizzy, vomiting and struggling to breathe, 11 Iraqi police officers were rushed to a government hospital 50 miles north of the capital last month. The diagnosis: poisoning by chlorine gas. The perpetrators, according to the officers: Islamic State extremists.

The chlorine attack appears to be the first confirmed use of chemical weapons by the Islamic State on the battlefield. An Iraqi Defense Ministry official corroborated the events, and doctors said survivors’ symptoms were consistent with chlorine poisoning.

Iraqi forces say two other crude chlorine attacks have occurred since the extremists seized vast tracts of Iraqi territory this summer, but details on those incidents remain sketchy. The reported assaults all raise concerns that the militants are attempting to hone their chemical weapons capabilities as they push to control more ground. […]