You know one thing late Thursday that the Islamic State used a rocket containing mustard gas against U.S. troops in Iraq. The general was addressing leaked news reports that investigators had found a black oily substance on a rocket that landed near the perimeter of the base. Dunford told the committee that investigators had assessed the liquid and found “it to be a sulfur-mustard blister agent.” The news is concerning because this is the first time a chemical weapon has been used against U.S.   officials said the Islamic State was using to produce mustard agent and other chemicals for military use. No soldiers were injured in the attack, nor have any shown symptoms of exposure to mustard gas, a weapon banned by the 1925 Geneva protocol after the horrors of World War I.