No 'mustard agent' on ISIS rocket that landed in northern Iraq, US military says

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In an about-face, U.S. military officials said Tuesday that no "mustard agent" turned up on the rocket that landed on a U.S.-controlled base in northern Iraq last week, according to lab tests.

The rocket, reportedly launched by Islamic State fighters, landed last Wednesday at the Qayara West air base, where hundreds of U.S. troops are working to advise and assist their Iraqi counterparts, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told Congress. He had initially told lawmakers that mustard agent was present on the rocket's shell.

A U.S. official, who revealed details to The Associated Press on condition of anonymity, said a small group of U.S. soldiers who inspected remnants of the rocket after it exploded found a black, oily substance on a fragment of metal. An initial test of the suspicious substance showed it contained residue of mustard agent, but a second test was negative.

On Monday, a Pentagon spokesman said ISIS has used chemical agents in the past and warned that the group might try again.

The U.S. military destroyed a large chemical weapons facility outside Mosul earlier this month.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.