Ruben Sprich / Reuters On Tuesday, ISIS took to its radio station to boast that the men who attacked the Muhammad Art Exhibit and Cartoon Contest in Garland, Texas, on Sunday night were "two soldiers of the caliphate." The claim, which has not yet been verified by any American officials, is the first attack on American soil for which the terror group has taken responsibility, but ISIS vowed it would not be the last: We tell America that what is coming will be even bigger and more bitter, and that you will see the soldiers of the Islamic State do terrible things.