Story highlights No US troops were hurt or have displayed symptoms of exposure to mustard agent

The first test was positive and the second was negative

Washington (CNN) ISIS is suspected of firing a shell with mustard agent that landed at the Qayyara air base in Iraq Tuesday where US and Iraqi troops are operating, according to several US officials.

The shell was categorized by officials as either a rocket or artillery shell. After it landed on the base, just south of Mosul, US troops tested it and received an initial reading for a chemical agent they believe is mustard.

No US troops were hurt or have displayed symptoms of exposure to mustard agent.

US officials said Tuesday that additional testing had definitely concluded that a mustard agent was not used in the attack.

Last week, another official had said the agent had "low purity" and was "poorly weaponized." A second official called it "ineffective."

Read More