Updated at 6 p.m. ET
FBI Director James Comey says the bureau issued a bulletin on one of the two assailants at a Prophet Muhammad cartoon contest in Garland, Texas, just three hours before the attack earlier this week.
Comey told reporters Thursday that the FBI had sent an Intel Bulletin to local law enforcement with a photo of Elton Simpson, his license plate number and other information without stating directly that he was heading to Garland.
He said the bureau had "learned hours before ... that [Simpson] was interested in the event but didn't know more than that.
"We developed information before the event [that] Simpson might go," Comey said, but added that before the event began, that the FBI "didn't have reason to believe he would attack."
On Sunday, Simpson and Nadir Soofi jumped out of a dark-colored vehicle and began firing assault rifles at the Curtis Culwell Center in Garland, where an anti-Muslim group was holding the contest.
A police officer returned fire with his service revolver and killed both men. Comey praised the officer for his "judgment, clearheadedness and skill."
The self-declared Islamic State later claimed responsibility for the attack.
As NPR's Dina Temple-Raston reported earlier this week, Simpson, an Arizona man who converted to Islam, was convicted of lying to federal authorities in 2011.
Simpson's arrest stemmed from an FBI investigation into whether he might travel to Somalia to wage jihad there. Comey said the bureau had reopened its investigation into Simpson in March based on new evidence that he was indeed interested in jihad — this time with the Islamic State.
Comey said the terrorists are using the "siren song" of Twitter and other social media to send the message that "if you can't travel, kill where you are." There are thousands of English-speaking followers on Twitter, with hundreds and maybe thousands in the U.S. "consuming this poison," he said.
The social media strategy being run out of the safe-haven of Syria has changed the paradigm for U.S. counterterrorism officials, he said. As a result, it may no longer be a useful distinction to think in terms of home-grown extremists. The old core of al-Qaida would do the vetting of people before approving them for missions, but that's no longer the case with the new offshoots, he added.
"It's almost as if there's a devil sitting on the shoulder saying 'kill, kill, kill,'" Comey said.
He said that ISIS is "a very popular fad among a lot of disturbed people."
On moving from Twitter to encrypted platforms, Comey said the FBI faces a challenge. "The haystack is the entire country now and here's the really hard part — increasingly the needles are invisible to us," he said.
"I know there are other Elton Simpsons out there," Comey said. "But I also know there are Elton Simpsons out there I cannot see."