PHOENIX (AP) — The Islamic State group claimed responsibility Tuesday for the assault on a Texas cartoon contest that featured images of the Prophet Muhammad, marking the first time the terror group has taken credit for an attack in the United States.

But it was unclear whether the group actually directed Sunday's shooting in the Dallas suburb of Garland or if the two gunmen were inspired by the group to act on their own before they were shot and killed.

Such lone wolf attacks pose a daunting challenge to law enforcement, and IS has a history of claiming responsibility for attacks in which it played no operational role, counterterrorism experts said.

Federal officials identified the gunmen as Elton Simpson and Nadir Soofi, both Americans who lived in Phoenix. They were described as amicable and quiet and were sometimes seen feeding stray cats outside their apartment complex. 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.

White House press secretary Josh Earnest said it's too early to say whether the Islamic State group played a role in the attack. He said U.S. officials are working aggressively to counter efforts by terrorists to use social media to radicalize individuals in the United States.

IS 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."

Federal investigators were looking for links to overseas terror groups, but as of Tuesday afternoon had not disclosed any connection or evidence to back up IS's claims of responsibility.

Authorities have not revealed whether Simpson and Soofi had any contact with IS or if the group was even aware of the deliberately provocative cartoon contest in Garland.

The shooting appeared to be another example of a "do-it-yourself" jihadist, the type 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.

Simpson and 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 at the attackers.

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 faced three years of probation and $600 in fines and court fees.

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.

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.

In December, for instance, Man Monis, an Iranian-born, self-styled cleric with a long criminal history, took 18 people hostage inside a Sydney cafe, forced them to hold up a flag bearing the Islamic declaration of faith and demanded he be delivered a flag of the Islamic State group. Monis and two hostages were killed.

"The debate after 9/11 was whether too many resources were being focused on New York, Washington and not in the small towns. Now that's very much a real issue because of the volumes of recruitment being done by the Islamic state throughout the United States, Europe, and Canada," Venzke said. "So attacks on the coffee shop, the small events like in Australia, those are the kinds of things that we're going to be seeing more of in the future unfortunately."

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Watson reported from San Diego.

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Associated Press writers Eric Tucker and Nancy Benac in Washington, Brian Skoloff and Paul Davenport in Phoenix and Jamie Stengle in Garland, Texas, contributed to this report.