An ISIS mustard gas attack against U.S. and Iraqi forces is suspected after a rocket or artillery shell hit the Qayyara Air Base near Makhmour south of Mosul, according to American military officials.
No U.S. troops were hurt or experienced any symptoms of being exposed to the mustard agent in the Tuesday attack, the officials told CNN.
One official said the agent was of "low purity" and "poorly weaponized," reported CNN. Officials said two field tests were conducted after the weapon landed at the airfield with the first testing positive for the mustard agent and the second testing negative.
American troops who investigated the shell went through decontamination showers as a precaution.
Mustard gas is a chemical agent that causes severe burning of the skin, eyes, nose, mouth and throat, said the Daily Mail. The chemical can be absorbed into the body through inhalation, ingestion or by coming into contact with the skin or eyes.
Mustard gas is recognized by its 'mustardy' odor, though the smell has also been compared to garlic, horseradish or sulfur. It causes blisters and irritation of the eyes, nose and mouth, said the Daily Mail.
CNN previously reported that the Islamic State had fired rounds of mustard agent at opponents in Iraq and Syria, but this attack was the first believed to be directed at U.S. troops.
Mosul has been under the control of the Islamic State since June 2014, but U.S. officials have hinted at an effort to retake the city possibly in the fall, said Politico. Iraqi and Kurdish military and paramilitary branches have been preparing for the push into Mosul with U.S support at the air base.