The U.S. military is testing to see if a chemical agent may have been used in a rocket attack in Iraq by Islamic State that came within hundreds of yards (meters) of U.S. forces but injured no one, a U.S. military official said on Wednesday.

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The rocket fell on Tuesday in an unpopulated area near the Qayyara West base, where hundreds of U.S. forces are working to prepare an airfield ahead of Iraq’s offensive to retake the city of Mosul from the radical group Islamic State, said the official, who spoke with Pentagon reporters on condition of anonymity.

A group of U.S. forces inspected the fragments afterwards and took a small sample of a suspicious “tar-like, black, oily” substance, which initially tested positive for mustard agent but then tested negative in a subsequent examination, the official said. Further tests were underway. The incident was first reported by CNN.

As a precaution, they underwent routine decontamination procedures, including showers, but did not display symptoms that would typically show up within 12 hours of exposure.

“It’s been more than 24 hours and we haven’t seen anyone who has any indication of blistering or anything like that,” the official said.

“There are absolutely no injuries. Nobody has been off their typical work schedule. It hasn’t impacted the mission in any way.”

The U.S. military has used air strikes to repeatedly hit Islamic State’s chemical weapons stores, and experts have warned that the militant group might use the agents during the upcoming Mosul offensive.

The United States has also observed repeated Islamic State attacks over the past year or so against Iraqi troops, Kurdish forces and Syrian soldiers, a second U.S. official said. But the official stressed there had not been such attacks against the U.S. forces in Iraq, who number at least 4,400.

“I don’t know of a case like this where it was proximate to U.S. forces like this before,” the second official said.

The first U.S. official described the group’s technical skills as crude when it came to mounting a chemical weapon attack and appeared to play down any growing concerns about a threat to U.S. forces.