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— 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 Islamic State 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.

Islamic State 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 the group's claims.

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

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 is considered blasphemous.

U.S. offers rewards

The Obama administration is offering rewards of up to $20 million for information leading to the whereabouts of four top leaders of the Islamic State group.

Through its Rewards for Justice program, the State Department announced Tuesday that it would pay up to $7 million for information on Abd al-Rahman Mustafa al-Qaduli, up to $5 million each for Abu Mohammed al-Adnani and Tarkhan Tayumurazovich Batirashvili and up to $3 million for Tariq Bin-al-Tahar Bin al Falih al-'Awni al-Harzi.

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