Members of the United Constitutional Patriots share cigarettes while patrolling the US-Mexico border in Sunland Park, New Mexico on March 20, 2019. (PAUL RATJE/AFP/Getty Images)

Armed vigilantes patrolling the U.S.-Mexico border have detained hundreds of migrants, some with young children, over the last week in the New Mexico desert.

A series of Facebook Live videos that surfaced this week show the men, from a militia group called the United Constitutional Patriots, rounding up groups of migrants — including many families with young children — at gunpoint and making them kneel on the ground. The militia then detains the migrants until Customs and Border Protection agents show up.

In the videos, the militia members shine their flashlights onto the group while making cruel comments. In one case, a woman who's with the group can be seen filming and saying: “Lots of coughing. How bad does it have to get until we build the wall. This is an invasion.”

Militia groups have patrolled the U.S.-Mexico border for decades. But as President Donald Trump ramps up his rhetoric against new migrants — he's called them “an invasion” and declared “our country is full” — the United Constitutional Patriots appears to have taken matters into their own hands.

In an interview last month with ABC-7 KVIA, one member of the United Constitutional Patriots said that CBP asked for their help at the border. CBP didn’t outright deny that.

“U.S. Customs and Border Protection does not endorse private groups or organizations taking enforcement matters into their own hands,” a CBP spokesperson wrote in an email to VICE News. “Interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved.”

To experts who have studied the militia movement, the United Constitutional Patriots' actions suggest a return to a dark period in American border history.

In the 1970s, the Ku Klux Klan formed vigilante groups in Texas and California with the goal of detaining undocumented immigrants and assisting CBP, according to Kathleen Belew, an assistant history professor at the University of Chicago and author of “Bring the War Home: The White Power Movement and Paramilitary America.”

In the 1980s, a militia group called the Civilian Military Assistant, equipped with semiautomatic weapons, ventured into Mexico, where they set booby traps and fired on groups of immigrants making their way to the United States, Belew found.

Vigilante groups have continued to patrol the border in recent decades, although in a less organized and paramilitary manner, compared to previous years. But the new development has troubled civil rights groups.

"We’ve never seen anything like this," said Peter Simonson, executive director of New Mexico’s ACLU. "This would always be our worst nightmare: that the vigilante groups would start falsely arresting people."

The national arm of the organization has now called on New Mexico’s governor and attorney general to investigate the militia’s border actions. “They have no authority under New Mexico or federal law to detain or arrest migrants in the United States,” the ACLU wrote in a letter.

New Mexico Attorney General Hector Balderas released a statement Friday urging the militia members to stop detaining migrants. "My office has been informed that this week, an armed group has detained nearly 300 people near Sunland Park, New Mexico," Balderas wrote. "These individuals should not attempt to exercise authority reserved for law enforcement."

“They are treating these folks as if they were cattle. Herding them up. Making them kneel in the sand. Even though there are children, infants, even,” Simonson added. “No compassion for the fact that they are holding innocent families at gunpoint.”

“Not a bunch of hillbillies”

For a report on the United Constitutional Patriots published last November, the Southern Poverty Law Center interviewed the militia’s “general,” Jim Peyton, who estimated the group had about 100 members. Peyton also said that every member had served in the military.

“We’re not a bunch of hillbillies running around with muskets,” Peyton told the SPLC. “People know what the rules are, what the rules of engagement are, what the rules of the border patrol are. And that’s how we’re operating.”

The SPLC’s report also zeroed in on how the United Constitutional Patriots felt emboldened by and promoted anti-immigration conspiracy theories — many of which Fox News hosts and Republican lawmakers, like Florida congressman Matt Gaetz, also shared.

For example, militia members advanced the baseless theory that Islamic State terrorists were embedded with the thousands of migrants traveling caravan-style toward the border. They also believe that billionaire George Soros bankrolled the caravan. The MAGA-Bomber, who last Novemberwaged a weeklong mail-bomb campaign targeting Trump’s biggest critics, also shared similar conspiracy theories.

The militia also produces a radio program that peddles anti-immigrant conspiracy theories, according to a report by the Daily Beast. The Beast also found evidence that the militia was pro-QAnon, a bizarre group of conspiracy theorists who believe that Trump and special counsel Robert Mueller were in cahoots to bring down a global child sex-trafficking ring run by Democrats.