ISIS claimed responsibility Tuesday for the assault on a Texas cartoon contest that featured images of the Prophet Mohammad, but counterterrorism experts said ISIS has a history of asserting involvement in attacks in which it had no operational role.
That suggests the two gunmen could have carried out their own lone wolf-style strike before they were shot and killed at the scene of Sunday's shooting in the Dallas suburb of Garland.
Federal officials identified the pair as Elton Simpson and Nadir Soofi, both Americans who lived in Phoenix. Federal authorities had been scrutinizing Simpson's social media presence recently but had no indication he was plotting an attack, said one federal official familiar with the investigation.
Rep. Michael McCaul, chairman of the House Homeland Security Committee, said a Twitter account linked to Simpson included images of Anwar al-Awlaki, a radical cleric killed in a CIA drone strike in Yemen. Al-Awlaki was born in the U.S.
Among the hashtags used by the account was "#texasattack." And one of the final tweets was: "May Allah accept us as mujahideen," or holy warriors.
"Was he on the radar? Sure he was," McCaul said from Turkey, where he was leading a congressional delegation.
The evidence does not indicate the attack was directed by ISIS, "but rather inspired by them," said McCaul, who was briefed on the investigation by federal law enforcement officials. "This is the textbook case of what we're most concerned about."
A federal law enforcement official told The Associated Press that authorities had an open investigation into Simpson at the time of the shooting. The official, who was not authorized to discuss the ongoing investigation by name and spoke on condition of anonymity, also said investigators will be studying the contacts the men had prior to the shooting, both with associates in the U.S. and abroad, to determine any additional terror-related ties.
White House press secretary Josh Earnest said U.S. officials are working to counter terrorist efforts to use social media to radicalize individuals in the United States.
ISIS recently urged those in the United States, Europe and Australia who cannot safely travel to fight in Syria and Iraq to carry out jihad in the countries where they live. An audio statement on the extremist group's Al Bayan radio station called the men "two soldiers of the caliphate."
The shooting appeared to be another example of a "do-it-yourself" jihadist whose plots are often hard for law enforcement and intelligence agencies to stop, said Mitchell Silber, executive managing director for K2 Intelligence and former director of intelligence analysis for the New York City police department.
"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 said.
The cartoon contest had been expected to draw outrage from the Muslim community. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad - even a respectful one - is considered blasphemous, and drawings similar to those featured at the Texas event have sparked violence around the world.
The 31-year-old Simpson and 34-year-old Soofi were wearing body armor, and one of the men shot a security officer in the leg before a single Garland police officer fired on the two gunmen. After his initial shots, nearby SWAT officers also fired, authorities said.
The security officer was treated at a hospital and released.
Simpson was arrested in 2010 after being the focus of a four-year terror investigation. But despite amassing more than 1,500 hours of recorded conversations, including Simpson's discussions about fighting nonbelievers for Allah and plans to link up with "brothers" in Somalia, the government prosecuted him on only one minor charge - lying to a federal agent. He was sentenced to three years of probation and ordered to pay $600 in fines and court fees.
The men were described as amicable and quiet and were sometimes seen feeding stray cats outside their apartment complex.
Simpson had worshipped at the Islamic Community Center of Phoenix for about a decade, but he quit showing up over the past two or three months, the president of the mosque told The Associated Press.
The center's president, Usama Shami, said Simpson would play basketball with mosque members and was involved with the community. Soofi owned a nearby pizza business and would stop in to pray occasionally, he said.
"They didn't show any signs of radicalization," Shami said.
Sharon Soofi, the mother of Soofi, told The Dallas Morning News that her son may have somehow snapped.
"The hard thing is to comprehend is why he would do this and leave an 8-year-old son behind," said his mother, who now lives in a small town southwest of Houston.
A University of Utah spokeswoman confirmed Tuesday that Soofi was an undergraduate pre-medicine major enrolled from fall of 1998 to the summer of 2003. Spokeswoman Maria O'Mara said Soofi did not earn a degree.
In a statement released late Monday by Phoenix law firm Osborn Maledon, Simpson's family said it is "struggling to understand" what happened.
"We are sure many people in this country are curious to know if we had any idea of Elton's plans," the statement said. "To that we say, without question, we did not."
Terrorism expert Ben Venzke, who has been tracking terrorist groups for two decades, said jihadists have shifted their tactics to include not only major targets - like al-Qaida's attack on the World Trade Center - but also small ones that are more easily accessible.