A rocket fired last week at an Iraqi base where American troops are present did not contain a chemical agent despite earlier suspicions, the U.S. military reported Tuesday.
Extensive laboratory tests concluded that the munition did not contain mustard agent, Air Force Col. John Dorrian, a U.S. military spokesman, said.
No one was killed or injured in the Sept. 20 attack. The shell landed several hundred yards from the nearest U.S. troops.
The improvised weapon appeared to have been crudely made and fired from a rocket launcher, the military said. It was one of a small number of shells that fell on the base, according to the U.S. military.
One initial field test proved inconclusive but another test uncovered traces of sulfur mustard, a dangerous and banned substance that can cause painful burns on skin and lungs if breathed in.
Joseph Dunford, chairman of the Joint Chiefs of Staff, told Congress last week the military assessed the munition contained sulfur mustard agent. His assessment was based on the information available at that time, the Pentagon said.
The substance was sent to labs for more extensive tests, a process that can take days.
The munition landed on Qayara West, an air base that was seized from the Islamic State recently and is serving as a staging area for the upcoming offensive to retake
Mosul, Iraq’s second largest city.
The Islamic State, which is battling U.S.-backed Iraqi and Kurdish forces, has used chemical weapons, and coalition aircraft have targeted a number of facilities where the militants have manufactured such munitions, including a large pharmaceutical facility in Mosul.
The Islamic State, also known as ISIS or ISIL, "continues to try and develop a chemical weapons capability," Dorrian said.
Islamic State militants will likely try to use chemical weapons as Iraqi forces launch an offensive to retake Mosul, Navy Capt. Jeff Davis, a Pentagon spokesman, said.
"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 said.
Militants there are building elaborate defenses, which include tunnels and moats filled with oil, which can be set ablaze. The Pentagon has estimated there are thousands of militants defending the city, which is considered an important part of the Islamic State's caliphate.
The Pentagon has said U.S. troops are trained and equipped to deal with chemical weapons. The U.S. military said it has also provided Iraqi and Kurdish forces with thousands of gas masks.