Hours after the Islamic State group claimed responsibility for Sunday’s attack on an exhibit of cartoons of the Prophet Muhammad in Garland, Texas, counterterrorism experts questioned the role of ISIS in the shooting.

Although a Twitter account traced back to Elton Simpson -- one of the two men who opened fire at the art exhibit and was later killed by police -- had claimed allegiance to ISIS in a tweet posted before the attack, there is reportedly no evidence to show that the attack was directed by the militant group. In all probability, the two men had carried out the shooting after being “inspired” by ISIS, Michael McCaul, chairman of the House Homeland Security Committee, told the Associated Press (AP).

“This is the textbook case of what we're most concerned about,” McCaul said.

ISIS has frequently called on its sympathizers in the U.S., Europe and other parts of the world, to carry out “lone-wolf” attacks in their countries. Although there is no evidence to show that Simpson and his roommate Nadir Soofi -- the second gunman who was also shot dead by police -- had ever travelled to ISIS camps in Syria or Iraq, Simpson’s social media presence makes it clear that he was an active ISIS sympathizer.

On May 3 -- the day of the incident -- a Twitter account named “Shariah is Light,” which was later traced back to Simpson, had hinted at the possibility of an attack, SITE Intelligence group reported. Using #texasattack as a hashtag, Simpson reportedly posted the following message: “The bro with me and myself have given bay’ah to Amirul Mu’mineen. May Allah accept us a mujahideen. Make dua.” Bay’ah is an oath of allegiance to a leader.

In the weeks and months leading up to the attack, Simpson had reportedly been posting pro-ISIS messages and retweeting links to ISIS sympathizers’ accounts. One of these accounts belonged to Mohamed Abdullahi Hassan, a Somali-American now believed to be in Somalia. 

In a tweet posted on April 23, Hassan urged “brothers in the U.S.” to “do their part,” SITE reported. In another tweet, Hassan posted photos of Amedy Coulibaly -- the gunman involved in a shooting at a supermarket in Paris in January -- and said: “If only we had men like these brothers in the #States, our beloved Muhammad would not have been drawn.” This message was reportedly retweeted by Simpson. Following the Texas attack, Hassan’s Twitter account was taken down.

Simpson, who was being tracked by the FBI, had been prosecuted in 2011 by the U.S. government for lying to federal officials about his plans to travel to Somalia and for making false statements related to "international and domestic terrorism." He was later sentenced to three years probation and a $500 fine.

Mitchell Silber, executive managing director for K2 Intelligence and former director of intelligence analysis for the NYPD, told AP that such lone-wolf attacks would remain extremely hard to detect and prevent. “It's very tough to detect in advance, which means we are and will continue to be susceptible to lone actors who don't give us much warning to thwart them,” he reportedly said.