ISIS claimed responsibility Tuesday for the attack at a Texas event featuring caricatures of the Prophet Muhammad — but did not provide evidence to support its claim.

Charlie Winter, an NBC News counter-terrorism consultant, confirmed that ISIS said in an official radio broadcast Tuesday that two of its “soldiers” had carried out the attack in Garland, Texas.

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Winter — who is also a researcher at the London-based Quilliam Foundation think tank — said the broadbast also warned that “worse” things are coming for America.

Roommates Elton Simpson and Nadir Soofi were shot dead on Sunday after allegedly opening fire with assault rifles outside the inaugural Muhammad Art Exhibit and Contest in the Dallas suburb of Garland.

The event offered a $10,000 prize for cartoons of the Islamic prophet.

This story originally appeared on NBC News