An audio statement on the extremist group’s Al Bayan radio station said “two soldiers of the caliphate” carried out the attack. It did not provide details and it was unclear whether the group was opportunistically claiming the attack as its own.
One of the suspected gunmen was known to the FBI since 2006, but despite more than 1,500 hours of surveillance, he was prosecuted only once, it has emerged. Agents recorded Elton Simpson talking about fighting non-believers for Allah, his plans to travel to South Africa and link up with “brothers” in Somalia and using school as a cover story for travelling overseas.
Simpson was arrested in 2010, a day before authorities say he planned to leave for South Africa. But the US government prosecuted him on only one minor charge – lying to a federal agent.
Years spent investigating Simpson for terrorism ties resulted in just three years of probation and $600 (£400) in fines and court fees.
On Sunday, two men whom authorities identified as Simpson and Nadir Soofi opened fire in the Dallas suburb of Garland on an unarmed security officer stationed outside the contest.
The deliberately provocative contest had been expected to draw outrage from the Muslim community.
Simpson, 30, of Phoenix, Arizona, and Soofi were wearing body armour, and one shot the security officer in the leg.
Garland police spokesman Joe Harn said a single police officer subdued the gunmen, but after his initial shots, SWAT officers nearby also fired at the two men. Mr Harn said police did not know who fired the lethal shots.
Homeland security secretary Jeh Johnson said authorities were investigating the men’s motives and all circumstances surrounding the attack.
“While all the facts are not in yet, last night’s attack serves as a reminder that free and protected speech, no matter how offensive to some, never justifies violence of any sort,” he said.
Simpson, described as quiet and devout, had been on the radar of law enforcement because of his social media presence, but authorities did not have an indication that he was plotting an attack, said an official familiar with the investigation.
Less was known about Soofi, who appeared to have never been prosecuted in federal court, according to a search of records.
His mother Sharon, who now lives in a small town south west of Houston, told The Dallas Morning News she had no idea that he would turn to violence.
She said her son was “raised in a normal American fashion” and “was very politically involved with the Middle East. Just aware of what’s going on”.
“I don’t know if something snapped,” she said.
She said the last time she communicated with her son was last month, sending a text to wish her grandson a happy birthday.
“He put his son above everything, I thought,” she told the newspaper. “The hard thing is to comprehend is why he would do this and leave an eight-year-old son behind.”
Federal agents spent hours on Monday searching a Phoenix apartment complex where the men apparently lived.