FBI sent alert about gunman before Texas shooting
The FBI had warned Texas officials several hours before a provocative Prophet Muhammad cartoon contest that a suspected extremist could show up at the event.
But FBI Director James Comey said on Thursday the agency had no indication that the attack on Sunday was planned.
Elton Simpson and Nadir Soofi were shot dead after they fired on a guard.
Simpson, who had social media ties to Islamic State, had alluded to an attack on Twitter.
US authorities have said Simpson had been under surveillance since 2006 and was convicted in 2010 for lying about plans to go to Somalia. Soofi was Simpson's roommate and had not been previous tied to terrorism.
The incident on Sunday unfolded when a car drove into the car park of the Curtis Culwell Center in Garland, a city near Dallas, where a Muhammad Art Exhibit organised by the controversial American Freedom Defense Initiative (AFDI) was being held.
Simpson and Soofi - both Phoenix residents - wounded a security guard before they were killed.
The conference included a contest that offered a $10,000 (£6,600) prize for a cartoon of the Prophet Muhammad. Depictions of the Prophet are offensive to many Muslims.
The FBI director said the attempted attack highlights the danger of home-grown extremists, who can gain inspiration and expertise from overseas militants via social media.
"The siren song sits in the pockets, on the mobile phones, of the people who are followers on Twitter," Mr Comey said. "It's almost as if there's a devil sitting on the shoulder, saying, `Kill! Kill! Kill! Kill!' all day long."
Mr Comey also spoke of differentiating those who talk of attacks online and those who will actually carry out violence.
"I know there are other Elton Simpsons out there," he said.