The FBI says that social media, specifically Twitter, was behind the plot to attack the draw Muhammad cartoon contest at Garland's Curtis Culwell Center on Sunday.
Lawmakers want to know how deep the ISIS internet connection is.
Long before Sunday's Garland shooting, the ISIS message had found its way into the heart and mind of at least one of the two gunmen.
"They are excellent at messaging," said Daveed Gartenstein-Ross, author of Home Grown Terrorists in the U.S. and U.K. "Technically, they go far beyond what Al-Qaeda and others have done…and they take advantage of Web 2.0: the interactivity of the internet, which suddenly makes someone who is alone a part of a group."
Recruiting begins openly with online posts like ones seen on the Middle East Media Research Institute's website.
"It can afford to have 2,000 people who tweet 150 times every day," said J.M. Berger, author of Jihadi Joe: Americans Who Go Fight for ISIS. "It can afford to have a ratio of, you know, two or three recruiters to every one potential recruit that might carry out a lone wolf attack."
Elton Simpson, one of the suspects killed Sunday in the Garland shooting, was reading those posts, investigators say.
"In the case of Garland, for instance, if we had been sweeping up those accounts, we would have much clearer idea of the track of radicalization for the suspect in open source," said Berger.
"They recruit, they talk, they talk online...then what happens?" said U.S. Sen. Ron Johnson.
"That can be a direct message on Twitter, which can't be read in the open source or Facebook," said Berger. "More often, they'll go to a more encrypted app such as WhatsApp or Kik, which allows you…it's basically text messaging with an element of encryption."
The committee was told that the U.S. has hurdles trying to keep up with violent extremism messaging.
"If you're monitoring 60 or 100 people, it takes 500 people to do that, to monitor these people even on a partial basis, let alone 24 hours a day," said Berger. "So if these guys jump in a car and drive to Texas, there's not a lot you can do to interdict that."
"The U.S. Government has to be more quick to react and to understand the strength of its messaging and to be able to respond at the same kind of speed focusing on the key message of the Islamic state at the same speed which they can push out their own message," said Gartenstein.