FBI Director James Comey said Thursday his agents learned hours before the start of a cartoon contest and exhibit depicting the prophet Muhammad that one of the gunmen had expressed interest in going to the controversial event in Texas, but there was no indication he was planning an attack.

Comey said the FBI sent an intelligence bulletin to local authorities through its Dallas field office that included a picture of Elton Simpson, 30, and other details such as his associates and possible license plate numbers.

"But we didn't know more than that," Comey said in an interview with reporters at FBI headquarters.

Simpson and his roommate, Nadir Soofi, 34, who lived in Phoenix, were killed by a local traffic officer as they opened fire at a conference center in Garland, Texas, where the competition was taking place.

The two men were heavily armed, U.S. law enforcement officials said. They had six weapons — four handguns and two semiautomatic assault rifles. Soofi is thought to have purchased one of the rifles on Craigslist, officials said.

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All of the weapons were purchased legally, according to officials who spoke on the condition of anonymity to discuss an ongoing investigation.

Comey also provided details about the FBI's long-standing interest in Simpson. He said FBI agents in Phoenix began investigating Simpson in about 2006, based on information that he was looking to travel to Somalia to join al-Shabab, an al-Qaida affiliate and a U.S.- designated terrorist organization.

Simpson was arrested in 2010 after he bought a plane ticket to South Africa. He was charged with lying to FBI agents in connection with terrorism. He was convicted, however, of a lesser charge the following year and sentenced to three years' probation.

Comey said the FBI continued to investigate Simpson until 2014. Agents in Phoenix reopened the case in March, Comey said, after "we developed information that he was making statements on social media that might indicate a renewed interest in jihad, but this time with" the Islamic State.

He added that the "investigation was open, but far from complete."

Comey declined to discuss whether the FBI was also looking at Soofi, but other officials said agents in Phoenix were preparing to investigate him when the shooting took place.

In an interview with the Wall Street Journal, Soofi's mother, Sharon Soofi, said her son had discussed Anwar al-Awlaki, the radical American preacher who took on a prominent role in al-Qaida's Yemen affiliate before he was killed in a 2011 drone strike.

Soofi's mother said her other son, Ali Soofi, lived with his brother and Simpson. She said Ali Soofi was concerned about Simpson's influence on his brother and recently moved out.

Comey said the FBI will review whether the bureau made any missteps and could have stopped Simpson and Soofi before the attack. But, he said, he hasn't seen any indications that his agents missed a major warning sign.

The roommates apparently fell under the sway of the Islamic State, which has been using social media, including Twitter, to encourage attacks in the United States.

Comey said the Islamic State's effort to recruit and motivate people in the United States to try to do harm was paying dividends.

"If you can't travel, kill where you are," Comey said, echoing the Islamic State's message to followers. He described the Islamic State's message as a siren's song, one easily accessed on phones or computers.

"It's almost if there is a devil sitting on the shoulder saying, 'Kill,' 'Kill,' 'Kill,' 'Kill,' all day long," he said.

Comey also discussed whether the attack was actually directed by the Islamic State. He said al-Qaida previously would train its operatives so they could be vetted before getting an assignment.

"But what we see [the Islamic State] doing is trying in a way to test people's bona fide by urging them to kill," he said.

"Sometimes supplying with them hit lists, sometimes suggesting particular targets, but other times just 'kill in our name.' In a way, the old paradigm between inspired or directed breaks down here.

"The distinction is irrelevant. I find it not to be a useful framework any longer."