WASHINGTON, D.C. -- The U.S. military says a rocket that landed within several hundred yards of U.S. troops at a base in northern Iraq may have contained mustard agent.

No one was hurt in the attack on Qayara West air base, according to a U.S. military official who discussed details on condition that he not be identified.

Hundreds of troops are working at that airfield, doing construction and site preparation, Central Command said in a statement to CBS News.

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The official said the attack Tuesday was militarily insignificant. He said a small group of U.S. soldiers who inspected remnants of the rocket found a black, oily substance on a fragment of metal. An initial test of the substance showed it contained residue of mustard agent, but a second test was negative.

A U.S. military official tells CBS News that the substance was in a powdered form, covered with oil, which is apparently what first drew the attention of U.S. service members.

The official said the U.S. has seen this type of thing before, and it was probably developed at the University of Mosul. It was “something we prepare for,” the official said.

The fragment has been sent to a laboratory for further testing. The individuals who found it went through routine decontamination procedures, none have shown symptoms.