Jim Michaels

USA TODAY

A shell that may have contained a chemical substance landed in a base in northern Iraq where some U.S. forces are helping Iraqi troops prepare for a major offensive against the Islamic State, two U.S. defense officials said Wednesday.

The shell was ineffective and no one was killed or injured in Tuesday's attack. Islamic State militants, who have previously been accused of using chemical weapons, are suspected of firing the shell, said the officials, who asked not to be named since they were not authorized to discuss the incident publicly.

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The shell was tested in the field twice by coalition troops, the officials said. One test indicated the presence of a mustard agent and the other test was negative. The black tar like substance will be sent to a laboratory for more extensive testing.

The shell, which may have been fired by artillery or rocket, landed in Qayara West, an airbase 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, from the militants. Coalition troops, including a small number of Americans, were several hundred yards from where the shell landed.

The officials said the attack would not change the “security posture” of coalition troops in the area.

CNN first reported the attack.