The Islamic State terror group (ISIS) Tuesday issued a claim of responsibility for Sunday's attack on a Texas cartoon contest featuring images of the Muslim prophet Muhammad.

The claim was made in an audio message on the group's Al Bayan radio station, based in the Syria city of Raqqa, which ISIS has proclaimed to be the capital of its self-proclaimed caliphate. It is the first time ISIS has taken credit for an attack on U.S. soil, though it was not immediately clear whether the group's claim was an opportunistic co-opting of a so-called "lone wolf" attack as its own.

The message described the shooting suspects as "two soldiers of the caliphate" and added "We tell America that what is coming is more bitter and harder and you will see from the soldiers of the Caliphate what harms you."

The message also said the contest, which was being put on by a group known for controversial rhetoric about Islam, "was portraying negative images of the Prophet Muhammad."

House Homeland Security Committee Chairman Michael McCaul, R-Texas, told Fox News Tuesday that the attack was terrorism and at the very least inspired by ISIS.

McCaul also said in the days leading up to the attack, a joint FBI and Homeland Security bulletin was circulated and security for the event had been ramped up as a result, in Garland understanding that it was a target.