FBI Director James Comey on Friday spoke to law enforcement officials around the country to ask for help in keeping an eye on suspected extremists.
The Garland attack has prompted the FBI to order counterterrorism investigators to review how they're keeping tabs on suspected extremists, and whether more invasive monitoring, more in-person interviews and more door-knocks are necessary.
"We're trying to shake the trees more aggressively," one U.S. law enforcement official said.
Elton Simpson, one of the two attackers shot and killed by a Texas police officer, was already on the FBI's radar for having expressed interest in the Texas event featuring cartoon drawings of the Muslim prophet Mohammed.
The FBI sent Simpson's picture and license plate, adding to a number of other people being watched, to Garland police about three hours before the attack. But the FBI had no idea he was planning an attack or even that he was en route from his home in Phoenix to Texas.
Comey told a group of reporters Thursday that the FBI faced technical difficulties in trying to keep tabs on the hundreds of people who appear to have more than just a passing interest in ISIS propaganda on social media.
He said that investigators often see public contact between Americans and ISIS propagandists in Syria, but then the communication goes offline to peer-to-peer encrypted methods.
Comey described social media apps accessible on mobile phones as constant reminders to ISIS sympathizers to try to carry out an attack.
"It's almost as if there is a devil sitting on the shoulder saying 'Kill, Kill, Kill, Kill!' all day long," he said.
In the case of Simpson, the FBI saw his publicly visible tweets and the suggestion that he communicated with one American jihadi in Somalia via direct messages on Twitter.