A U.S. base near Mosul, Iraq, was possibly attacked with a chemical weapon on Tuesday, a U.S. official said Wednesday. The base outside Qayyarah came under rocket fire from Islamic State militants, and it’s believed that one of the shells contained traces of a “mustard agent.” An initial test was positive, and a second test came back negative, but that result could have been due to the shell being exposed to the elements. No one was injured in the attack, and no troops have shown any “signs of exposure.” A few soldiers who were close to the shell received full decontamination treatment, and residue from the shell is being sent to Maryland for testing. Mustard gas can spread, while mustard agent leaves behind an oily residue and is not as effective. The base, 25 miles south of Mosul, regularly comes under fire with conventional weapons, the U.S. official said, and is home to hundreds of the nearly 5,000 U.S. troops now stationed in Iraq.