The Islamic State has reportedly claimed responsibility for Sunday’s attack by Elton Simpson and Nadir Soofi on an anti-Muslim cartoon contest in Garland, Texas.

“Two soldiers...of the caliphate attacked an exhibit in Garland in American Texas, and this exhibit was holding a contest for drawings offensive to the Prophet Muhammad,” the group reportedly said on the group’s Al Bayan radio station in Raqqa, Syria, Tuesday. “We tell...America that what is coming will be more grievous and more bitter and you will see from the soldiers of the Caliphate what will harm you, God willing.”

The radio broadcast has yet to be independently verified.

Simpson and Soofi opened fire on a Garland traffic officer and a Garland Independent School District security officer outside of the Curtis Culwell Center just before 7 p.m. local time Sunday. A contest being held inside—of cartoons depicting the Prophet Muhammad, considered sacrilegious in the eyes of many Muslims—was about to end. The two men were shot and killed by the traffic officer at the scene; the security guard suffered a non-life-threatening gunshot wound.

As the Associated Press notes, while this would ostensibly be the first time ISIS has claimed responsibility for an attack on U.S. soil, the extremist group has provided no clear evidence that they either planned the shooting and the two men acted as their agents, or the two men simply pledged their allegiance to the group and carried out the attack on their own.

But at least one of the men had been accused of jihadist loyalties before. Simpson, the New York Times reports, was convicted in 2011 for lying to the FBI about having travelled to Somalia. Prosecutors claimed Simpson had gone to the country “for the purpose of engaging in violent jihad,” but a judge denied the charge, citing insufficient evidence, and sentenced Simpson to three years probation.

But the nonprofit Middle East Media Research Institute also confirmed to the Times Monday that a Twitter and other social media accounts belonging to one of the gunman (likely Simpson) had made contact with known ISIS supporters:

About the time of the attack Sunday, on a Twitter account with the name “Shariah is Light” that has since been suspended, someone posted using the hashtag #texasattack. The profile picture on the account is of Anwar al-Awlaki, a militant imam killed in a 2011 American drone strike in Yemen.

Mr. Awlaki repeatedly called for violence against cartoonists who, in his view, insulted the Prophet Muhammad. The Twitter post says that the writer and the man with him have “given bay’ah,” or pledged loyalty, “to Amirul Mu’mineen,” a title meaning commander of the faithful that was used by early Muslim rulers and has been claimed by Abu Bakr al-Baghdadi, leader of the Islamic State. “May Allah accept us as mujahedeen.”

A police source confirmed the suspicion that the Twitter account belonged to Simpson, telling the paper, “That’s certainly what we believe at this point.”

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