Syria (BBN)-ISIS has claimed responsibility for the attack outside the Prophet Mohammed cartoon contest in Garland, Texas -- and warned of more attacks to come
In a broadcast on its official radio channel Tuesday, the group said two Al Khilafa soldiers attacked the event. Al Khilafa is how ISIS refers to its soldiers, reports CNN.
The two gunmen, Elton Simpson and Nadir Soofi, wounded a security guard before police shot and killed them.
The ISIS radio announcer also referred to Simpson and Soofi as the terror group's "brothers."
The announcement ended with this warning:
"We say to the defenders of the cross, the US, that future attacks are going to be harsher and worse. The Islamic State soldiers will inflict harm on you with the grace of God. The future is just around the corner."
While ISIS claimed responsibility two days after the Sunday attack, there was no immediate indication that the terror group in Iraq and Syria actually had contact with Simpson or Soofi, who both lived in Phoenix.
US authorities have said they are investigating whether Sunday's shooting has any link to international terrorism.
But there are clues that one of the gunmen was an ISIS sympathizer.
Moments before the attack, Simpson posted an ominous tweet with the hashtag #texasattack: "May Allah accept us as mujahideen."
The tweet also said he and his fellow attacker had pledged allegiance to "Amirul Mu'mineen," which means "the leader of the faithful." CNN terrorism analyst Paul Cruickshank said that likely refers to ISIS leader Abu Bakr al Baghdadi.
And earlier, Simpson had asked his readers on Twitter to follow an ISIS propagandist.
After the shooting, the propagandist tweeted: "Allahu Akbar!!!! 2 of our brothers just opened fire."
Both Twitter accounts have been deactivated.
In 2011, Simpson was convicted of making a false statement involving international and domestic terrorism. Prosecutors said he told FBI agents that he had not discussed traveling to Somalia to engage in "violent jihad" -- when, in fact, he had, according to an indictment.
Simpson was sentenced to three years of probation, court records show.
Soofi, on the other hand, was relatively unknown to federal investigators, a law enforcement official told CNN.
Authorities knew of no indication the two planned to launch Sunday's attack, another law enforcement official said.