On America's Newsroom Monday, Judge Andrew Napolitano said that President Trump cannot permanently shut down the southern border through legal means.

Trump tweeted that if needed, his administration would close off the border after migrants from the Central American caravan were seen pushing past Mexican police toward the San Ysidro, California, point of entry over the weekend.

Napolitano said that the president "cannot legally" shutter the southern border due to federal statues already in place.

"That's the law as he finds it and that's the law as he's required to enforce it."

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Napolitano noted that the law was also referenced by U.S. District Judge Jon S. Tigar, whom President Trump called an "Obama judge" after he halted the president's plan to refuse asylum to illegal immigrants.

"[Trump] doesn't have to tolerate lawlessness and certainly the officials at the border can use reasonable resistance to lawlessness," he added. "But the law says if you have a bona fide asylum claim ... you can enter the United States."

He also said that Border Patrol agents have acted within the law with their use of tear gas against migrants.

Border Patrol Chief Carla Provost said earlier Monday on Fox & Friends that her agents were being pelted with rocks and debris, which prompted them to fire the gas.

"Their job is to protect themselves, to protect their colleagues and to protect the border in that order," Napolitano said. "Then, to protect the migrants."

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