ISIS claims responsibility for Texas attack

Last Updated May 5, 2015 4:45 PM EDT

The Islamic State of Iraq and Syria (ISIS) claimed responsibility Tuesday for the attack on a Texas art exhibit and contest featuring drawings of the Prophet Muhammad. It was the first attack ISIS has claimed inside the United States.

The terror group's official radio station made the claim without giving any details, and U.S officials have yet to confirm any actual involvement by the group. ISIS claimed in its statement Tuesday that "what's yet to come will be worse and more bitter," warning that the group would attempt further attacks in the U.S.

On Tuesday, White House spokesman Josh Earnest said "it's too early to say" whether U.S. officials have been able to corroborate claims that ISIS is responsible for the shooting. Earnest said that ISIS and others were trying to capitalize on social media to radicalize people in the U.S., adding that U.S. officials are aggressively working to counter those efforts.

CBS News correspondent Jeff Pegues reports Elton Simpson, one of the two gunman killed at the Texas art contest Sunday night, had previously expressed support for the terror group online and was under FBI watch long before the attack.

Law enforcement said there was no indication he was plotting violence, but a Twitter account often sympathetic to ISIS condemned the cartoon contest and warned of an attack just minutes before the shooting took place.

Simpson first surfaced in 2006 when the FBI began investigating his association with a person believed to be setting up a terror cell in Arizona.

There are recorded conversations between Simpson and an informant during which the two frequently discussed waging a holy war and the importance of going overseas to wage jihad. At that time, investigators believed Simpson wanted to go to Somalia to fight.

Court documents recount a conversation in May 2009 in which Simpson is quoted as saying, "It's time to go to Somalia, brother." "We gonna make it to the battlefield... it's time to roll."

In early 2010, FBI agents stopped Simpson from traveling to South Africa where he was allegedly going to study Islam. But according to court documents, investigators believed that his goal was actually go to South Africa and then on to Somalia. That was something Simpson denied. In 2011, he was found guilty of making materially false statements to the FBI and sentenced to three years' probation and a $500 fine.

The other gunman, Nadir Soofi, was not on law enforcement agencies' radar screens prior to the attack, says Pegues.

Soofi's mother, meanwhile, said Monday that she couldn't imagine what went through the mind of her son.

Sharon Soofi said her son was always deeply religious, but she never thought he would hurt anyone, reports CBS News correspondent Omar Villafranca.

"He was a practicing Muslim, but not in the extreme sense," Soofi said. "He just tried to make people understand Islam."

Authorities said Soofi, 34, and Simpson, 30, arrived Sunday at the Dallas-area community center armed with body armor, assault rifles and a mission.

"They were there to shoot people," Garland Police Department public information office Joe Harn said. "We will continue to investigate. This is not going to be a real fast investigation."

Soofi's mother believes Simpson, her son's roommate, orchestrated the attack.

"He's an intelligent kid. I mean, but to be convinced to do something like this is beyond, is just beyond me," Soofi said.

Federal agents spent hours Monday combing through the Phoenix apartment that the men shared, looking for answers.

Back in Texas, authorities credited Bruce Joiner, a security officer normally assigned to traffic duty, with thwarting their plan.

"He did a very good job and probably saved lives," Harn said.

Soofi's mother said she doesn't hold the officer responsible.

"He was just doing his job. But when your child dies under these kinds of circumstances it just leaves you numb and empty," she said.

Simpson's family is also struggling to understand how this could have happened.

"We are sure many people in this country are curious to know if we had any idea of Elton's plans. To that we say, without question, we did not," they said in a statement.