FBI and intelligence community officials are openly admitting how difficult it has become to track the thousands of potential American jihadists on social media.

Rep. Michael McCaul (R-TX), who serves as chairman of the House Homeland Security Committee, said in a recent hearing that there are “thousands” of people living in the U.S. who have promoted extremist groups, such as the Islamic State, on social media pages, the Associated Press reports.

The new concerns about tracking social media stem from a Sunday jihadist attack at a “Draw Muhammad” free speech event in Garland, Texas, where two gunmen armed with AK47s attempted to commit mass murder against the event’s attendees. Luckily, a skilled security officer neutralized the two men, Elton Simpson and Nadir Soofi, before they could do any real harm.

There are “hundreds, if not thousands,” of people like Simpson and Soofi whom the intelligence community are attempting to track, explained Don Borelli, a 25-year veteran in FBI terrorism investigations, to an Arizona NBC affiliate.

White House Press Secretary Josh Earnest said the intelligence community has opened up an investigation into the Islamic State’s claim that it ordered the attack.

“This is consistent with what has been previously described as a lone-wolf attack,” Earnest told reporters. “Essentially you have two individuals that don’t appear to be part of a broader conspiracy, and identifying those individuals and keeping tabs on them is difficult work,” he added.

The Islamic State appears to have endorsed plots against the U.S. on a much smaller scale than Americans are used to. When we think of jihadi attacks in America, explains the AP, most think of the mass casualty attacks carried out by al-Qaeda.

“If you can get your hands on a weapon, how is the state security apparatus supposed to find you?” Will McCants of the Brookings Institute told the AP. “It’s attractive because it gets just as much attention as a small-to mid-size bomb.”

William Branniff of the terrorism research center at the University of Maryland added:

Where is the threshold of saying this is more than just an avid consumer of propaganda? It’s exceptionally difficult to estimate of the number of people who’ve considered becoming foreign fighters. Often you’re not dealing with specific behaviors, but with expressions of belief, which are constitutionally protected.

U.S. officials believe around 180 Americans have left the country to fight for what they perceive as Islamic holy wars in Iraq and Syria. The Islamic State has successfully recruited a plethora of American recruits, unlike some of their Sunni rivals.

“What is new is the level of excitement among extremists,” David Benjamin, former coordinator of counterterrorism at the State Department, told the AP. “The feeling is that ISIS has done what [al-Qaeda] couldn’t – it has held territory, it has damaged armies much larger than it is.”