Homeland Security Secretary Jeh Johnson acknowledged on Thursday that he's not completely sure the government is able to track every American who travels to and from the Middle East to fight with or otherwise aid the Islamic State.
"I have reasonable confidence that we're tracking these people," Johnson told CNN. "Do I have complete confidence? No, I can't say that to the American public, but I have reasonable confidence that we're tracking these people."
Johnson said that one of the problems the government has is tracking people who take indirect routes from the U.S. to meet up with the terrorist group.
"It's difficult to pick up so-called broken travel… where you fly to country A, and then you go to country B on the ground," he said.
"There's always an unknown factor," he said. "But I think we have, between Homeland Security, the FBI and the Department of Justice, and our intelligence community, systems in place to track these individuals either through travel information or through our intelligence collection efforts."
Johnson participated in this week's White House initiative to curb violent extremism, and told CNN that the Islamic State is having more success radicalizing Americans through social media.
"We're seeing more and more, through propaganda on the Internet, the ability of these terrorist organizations to reach into our communities and recruit people, and that's something that we're addressing very aggressively right now," he said.