The U.S. military confirmed Tuesday that a rocket, which landed at the Qayyara Air Base in Iraq last week where U.S. and Iraqi troops are operating, did not contain a chemical agent, according to the Military Times.
Lab tests concluded that the munition did not contain mustard agent, Air Force Col. John Dorrian told the Military Times.
No one was injured in the Sept. 20 attack despite the fact that the shell landed several hundred yards from U.S. troops.
Last week, an official told CNN the agent had "low purity" and was "poorly weaponized."
Initially, U.S. troops tested it and received a reading for a chemical agent believed to be mustard. Mustard is a banned substance that can cause painful burns on the skin and can damage lungs if it's inhaled.
However, further testing concluded that a mustard agent wasn't used in the attack.
A third test was inconclusive so further tests are being conducted.
U.S. troops involved in the incident went through a decontamination process just as a precaution.
So far, no troops have shown any signs of exposure to the chemical agent.
The Islamic State, which is battling U.S.-backed Iraqi and Kurdish forces, has previously fired rounds with mustard agents in Iraq and Syria.
Dorrian said ISIS "continues to try and develop a chemical weapons capability."
Navy Capt. Jeff Davis told the Military Times ISIS will likely try to use chemical weapons to retake Mosul, a key city in Iraq.
"They're dead set on it. They would love to be able to use chemical weapons against us, against the Iraqis, as they move forward," he added.
Previously, the Pentagon has said that U.S. troops are prepared to deal with chemical weapons and has already provided gas masks for Iraqi and Kurdish forces.
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