The Islamic State appears to have manufactured rudimentary chemical warfare shells and attacked Kurdish positions in Iraq and Syria with them as many as three times in recent weeks, according to field investigators, Kurdish officials and a Western ordnance disposal technician who examined the incidents and recovered one of the shells.

The development, which the investigators said involved toxic industrial or agricultural chemicals repurposed as weapons, signaled a potential escalation of the group’s capabilities, though it was not entirely without precedent.

Beginning more than a decade ago, Sunni militants in Iraq have occasionally used chlorine or old chemical warfare shells in makeshift bombs against American and Iraqi government forces. And Kurdish forces have claimed that militants affiliated with the Islamic State, also known as ISIS or ISIL, used a chlorine-based chemical in at least one suicide truck bomb in Iraq this year.

Firing chemical mortar shells across distances, however, as opposed to dispersing toxic chemicals via truck bombs or stationary devices, would be a new tactic for the group, and would require its munitions makers to overcome a significantly more difficult technical challenge.