The head of the U.S. Border Patrol called illegal immigration a national security threat, saying that the high number of known gotaways keeps him “up at night.”

Speaking with CBS News’s Face the Nation on Sunday, Border Patrol Chief Jason Owens warned that his agency is sometimes stretched thin, with 20,000 agents protecting nearly 2,000 miles at the southern border at all hours.

“[Cartels] dictate what the flow is going to look like,” Owens said. “We respond to it, and 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.”

Cartels will often send a group of migrants across the border, wait for border patrol to respond, and then send another group a few miles down the road, Owens said.

The biggest threat, he said, is to national security.

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

Former president Donald Trump has made illegal immigration one of the centerpieces of his 2024 campaign, saying that President Joe Biden has made the United States a “dumping ground” for other countries.

Owens said that people from at least 160 different countries have crossed the border illegally this fiscal year, from parts of Europe, Africa, and Asia in addition to the Americas. The United Nations recognizes a total of 193 member countries in the world.

Trump has also alleged that other countries are emptying mental institutions, jails, and prisons to send people across the U.S. border. Owens said that what scares him is not knowing whether that’s true or not.

“I don’t know if other countries are releasing people from jails, and those folks that got released are making their way up or not,” he said. “I don’t know what the numbers would be. It’s the unknown that that scares me.”

Biden has countered blame for the border crisis by saying Republicans tanked a border security bill in order to preserve the issue for Trump’s campaign, though Republicans pointed to specific policy provisions in explaining their opposition to the law.

Owens did say that he felt the vast majority of people crossing the border are “good.”

“I think they absolutely are by and large good people,” Owens said. “I wish they would choose the right way to come into our country and not start off on the wrong foot by breaking our laws.”

“It’s just that they’re not being respectful of the laws that we’ve established as a country, and they’re actually putting the people in this country in harm’s way because they’re pulling the border security apparatus off of task,” he said.

The number of illegal immigrants encountered at the nation’s borders in February remained at the highest levels seen in national history, increasing slightly to more than a quarter million people encountered.