The development, which investigators said involved toxic industrial or agricultural chemicals fired in Iraq and Syria, signaled a potential escalation of the group’s capabilities.

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The Islamic State group appears to have manufactured rudimentary chemical-warfare shells and attacked Kurdish positions in Iraq and Syria in recent weeks, according to field investigators, Kurdish officials and a Western ordnance-disposal technician who recovered one of the shells.

The development signaled a potential escalation of the group’s capabilities, though it was not without precedent.

Sunni militants in Iraq have occasionally used chlorine or old chemical-warfare shells in makeshift bombs against U.S. and Iraqi government forces. Kurdish forces have claimed that militants affiliated with the Islamic State group used a chlorine-based chemical in at least one suicide truck bomb in Iraq this year. Firing chemical mortar shells across distances, however, would be a new tactic for the group, and would require its munitions makers to overcome a significantly more difficult technical challenge.

In the clearest recent incident, a 120-mm chemical mortar shell struck a Kurdish position near the Mosul Dam on June 21 or 22, investigators said, and caused several Kurdish fighters to become ill.

The shell was recovered nearly intact June 29 by Gregory Robin, a former French military ordnance-disposal technician. The tail of the shell had been broken, Robin said, and was leaking a liquid that emanated a powerful odor of chlorine and caused irritation to the airways and eyes.