Department of Homeland Security Secretary John Kelly admitted on Sunday that he doesn’t know how to stop “homegrown terrorists,” which he described as the “most common” threat facing the United States. Kelly made the stark admission during an interview on CBS’s Face the Nation when he was asked about last week’s attack in Paris.

“There are so many aspects to this terrorist thing,” Kelly said. “Obviously you got the homegrown terrorists. I don’t know how to stop that. I don’t know how to detect that. You got other terrorist threats that come across the border. I believe in the case of the murder, in the Paris shooting I believe he was homegrown. But, again, there are so many threats that come in from across border. And it’s essential absolutely to control one’s border.” The reason Kelly emphasized the border so much is because he was advocating for Congress to fund the wall along the Mexican border.

When host John Dickerson asked him to clarify his statement, Kelly did acknowledge that homegrown terrorism is “a big threat” to the country. “Is it the No. 1 threat? I think it’s the most common threat.” he said.

So how can he help combat it? “I think the appeal I would make on the homegrown threat is if you see something, say something.”

Kelly also said that what keeps him “literally awake at night is the threat against aviation.” A successful attack on a plane “would be the Super Bowl for the terrorists,” and there are “a number of plots that we’re watching very, very closely.”

