U.S. Border Patrol Chief Jason Owens said over the weekend that the hundreds of thousands of illegal aliens who have evaded capture and made it into the U.S. homeland keep him up at night and that the situation is a “national security threat.”

Owens made the remarks during a Sunday interview with CBS News’ “Face The Nation” while talking about how Mexican drug cartels effectively set the rules of engagement on the southern border.

“They dictate what the flow is going to look like, and we respond to it,” he said. “Then we try and get out in front of it and deny them the ability to use these areas, especially ones that we think are going to be dangerous for us and for the migrants. But at the end of the day, there’s over 1,900 miles of border with Mexico. Now, when you talk about 20,000 border patrol agents, that sounds like a lot. But when you multiply that by 24 hours a day, seven days a week, across the entirety of the year, that number starts to dwindle very fast. And that’s exactly what they’ll do. The tactic is, they’ll push groups across knowing that we’re going to respond from a humanitarian perspective and make sure that they’re safe. And while we’re tied up and occupied doing this, what are they doing a couple miles down the road?”

He said that the entry of unknown foreign nationals into the U.S. is the top thing that keeps border officials up at night.

“We’re closing in on a million entries this fiscal year alone,” he said. “That number is a large number, but what’s keeping me up at night is the 140,000 known gotaways.”

“That is a national security threat,” he said. “Border security is a big piece of national security. And if we don’t know who is coming into our country and we don’t know what their intent is, that is a threat. And they’re exploiting a vulnerability that’s on our border right now.”

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