The two gunmen who attacked an anti-Islamic event on Sunday drove from Phoenix to Garland, Texas, before opening fire on police.
The gunmen also left a terror trail on the Internet.
Since Sunday, the ISIS propaganda machine has been working overtime praising the failed attack in Garland.
Online, a suspected American ISIS member proclaimed there are more attacks to come, with 71 so-called "trained soldiers" in 15 states.
Another ISIS member, from Britain, wrote, "you ain't seen nothing yet."
Elton Simpson, one of the gunmen killed Sunday, had exchanged messages with that man and also with Mujahid Miski, an American from Minnesota who is now believed to be a member of the terrorist group al-Shabaab in Somalia.
Just last month, Miski applauded the January attack on a French magazine in Paris, tweeting, "It's time for brothers in the #US to do their part."
Simpson tweeted back, "When will they ever learn? They are planning on selecting the best picture drawn of Rasulullah in texas," a reference to the contest for cartoons depicting the Prophet Muhammad.
Elliot Zweig with the Middle East Media Research Institute said terrorists are using social media to alert followers to locations for potential attacks.
"For the most part, it's encouragement of attacks," said Zweig. "Here is a target. Here is what the Westerners are doing. Here is a potenital place to focus efforts."
The FBI did not consider Elton Simpson and Nadir Soofi, the other gunman, "high priority" cases. One official said he's concerned about others like them "beneath the surface."
Social media is creating a gray area for law enforcement over whether ISIS specifically directed an attack.