A militia was ‘detaining’ migrants at the border. The FBI arrested its leader.

The FBI has arrested the leader of an armed militia that scours the southern border for undocumented migrants, days after a video of the group holding migrants against their will sparked outrage.

Larry Mitchell Hopkins, 69, was arrested Saturday in the New Mexico border city of Sunland Park on charges of being a felon in possession of firearms and ammunition, the FBI said.

He is expected to appear Monday in federal court.

Hopkins leads the United Constitutional Patriots, or UCP, one of several militias that have taken to patrolling the border. The patrols have been prompted by a recent surge in Central American migrants and emboldened by President Donald Trump's assertion that the migrants constitute an "invasion." The group's listed objective is to "uphold the Constitution of The United States of America" and to protect citizens' rights "against all enemies both foreign and domestic" - which mimics the Oath of Enlistment taken by U.S. service members.

In an April 16 video, posted by a woman who goes by Debbie Collins Farnsworth on Facebook, a large group of migrants, including several children, were seen sitting on the ground and huddled together in the darkness, some of their faces illuminated by flashlights.

"This is crazy, everybody, totally crazy," a voice that appeared to be Farnsworth's narrated as she walked the perimeter of the group, claiming that it consisted of "hundreds" of people. "I don't know what to say about this, other than the fact it's got to stop."

A few minutes into the 45-minute live video, Border Patrol officers arrived and ordered the migrants to sit down before telling them to begin walking. The migrants were shepherded through the night by Border Patrol officials while Farnsworth followed behind.

At one point, while watching adults walk past holding children's hands, she said: "See the way they hold their kids? I don't think those are their kids, honestly. They've got grips on their wrists. It's crazy."

Farnsworth's video had garnered more than 100,000 views and 2,700 shares by Sunday afternoon.

The militia has maintained that its actions are legal, though Sunland Park Police Chief Javier Guerra told BuzzFeed News he had explicitly informed the group that they are not.

On Thursday, the New Mexico chapter of the American Civil Liberties Union sent a letter to Gov. Michelle Lujan Grisham, D, and Attorney General Hector Balderas, D, demanding they investigate the UCP and the actions portrayed in the April 16 video.

"We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum," the letter said. "We urge you to immediately investigate this atrocious and unlawful conduct."

"Law enforcement belongs strictly in the hands of trained professionals," Peter Simonson, director of the ACLU in New Mexico, told The Washington Post. He said his group alerted officials because of fears that the armed militia members would harm the migrants.

On Friday, Grisham called the militia's actions "absolutely unacceptable."

"This is a dangerous felon who should not have weapons around children and families," Balderas said in a statement after Hopkins's arrest. "Today's arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not vigilantes."

Hopkins found himself in a similar situation in 2006 in Klamath County, Oregon, where he was arrested and charged with impersonating a police officer and being a felon in possession of a firearm, according to an incident report cited by the Southern Poverty Law Center. The report states that Hopkins was seen at a gas station in Keno, Oregon, near the California border, showing firearms to a group of children, telling them that he was a police officer.

A Klamath County sheriff's deputy wrote in the incident report at the time that he had "observed that Larry Hopkins was wearing a black uniform style shirt and black pants. Hopkins had a badge similar in appearance to a police officer badge pinned above his left breast in the area a police officer would wear a badge. Hopkins had a gold star on each of his collars which is often a sign of rank. Hopkins had several military or law enforcement style pins all over his shirt in a uniform appearance."

Hopkins was indicted on one count of impersonating a peace officer and two counts of being a felon in possession of a firearm but, in the end, he did not serve a sentence, according to the Southern Poverty Law Center.

Following Hopkins’s arrest over the weekend, one of the militia’s members, Jim Benvie, addressed the recent charges in a Facebook Live video, saying the UCP was not breaking any laws and that Hopkins would be exonerated.

He spoke as the camera remained pointed forward and asserted that Hopkins was "set up" by Grisham and Balderas based on the ACLU's "false, baseless allegations against the group."

"This was their first attack," Benvie said. "They want to create a narrative that there's a bunch of reckless criminals out on the border."

The UCP did not respond to a Facebook message from The Post requesting comment and did not respond to a phone inquiry.

A U.S. Customs and Border Protection spokesman distanced the agency from the militias in a statement to The Post, saying the agency "does not endorse private groups or organizations taking enforcement matters into their own hands" and warning that "interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved."

But some of the UCP’s videos showed Border Patrol agents arriving to take away migrants who had been stopped by militia members. One of its members, Mark Cheney, told BuzzFeed News that Border Patrol officers are “happy we’re here. . . . We have a direct line to the local outpost.”