Members of the militia group United Constitutional Patriots near the southern border in Sunland Park, N.M., on March 20. (Paul Ratje /AFP/Getty Images)

Kristy Parker, former deputy chief in the criminal section of the Justice Department civil rights division, is counsel with Protect Democracy, where Cameron Bills is a paralegal.

The leader of the militia group United Constitutional Patriots was arrested April 20 by the FBI on a federal weapons charge. As The Post reported, “The United Constitutional Patriots came to public attention this month after the emergence of videos that showed men stopping and detaining people crossing the border.” The videos included two that, CNN reported, appeared to feature a UCP member impersonating a Border Patrol agent. Another video showed armed men in military-style uniforms detaining dozens of migrants. “Customs and Border Protection agents arrive and collect migrants but do not ask the group with guns to disperse or take their weapons elsewhere,” The Post reported.

Vigilantism is always a threat to democracy, but the militia videos are especially troubling because of the appearance that civilians were masquerading as law enforcement officials — in at least one, according to The Post, members could be heard shouting “Policia, alto!” or “Police, stop!” — and because federal agents encountered the vigilantes without confronting them.

The apparent cooperation of federal border agents and civilian militias is a disturbing sign that President Trump’s open disregard for what he calls America’s “very stupid” immigration laws is taking hold in perilous ways. As revealed in the documents charging UCP leader Larry Mitchell Hopkins as a felon possessing firearms and elsewhere, the federal government is well aware that some militia members have dangerous criminal histories. (Hopkins’s lawyer said he will plead not guilty at a bond hearing next week.) A fusion of vigilantes and the state is a particular threat to the Constitution and one of the signs of rising authoritarianism that historians warn democracies to guard against.

The FBI’s arrest of Hopkins is encouraging, as is the recent condemnation of vigilante justice issued by New Mexico Gov. Michelle Lujan Grisham (D), but the activities of UCP and other militias on the border bear further investigation and action.

It is a federal crime, for instance, for militias to impersonate and act as federal law enforcement officers. Militias making “arrests” implicates other federal criminal laws, as well. And to the extent the Department of Homeland Security or individual Border Patrol agents collaborate with militias, they also violate federal law. The due process clause of the Fifth Amendment prohibits the government from subjecting private citizens to a state-created danger, as they would by partnering with — or even knowingly permitting — vigilantes to detain people and make arrests. The Fourth Amendment prohibits unreasonable seizures, and the Supreme Court has held that involving private citizens in arrests is indeed unreasonable.

Customs and Border Protection has issued a statement saying the agency “does not endorse or condone private groups or organizations taking enforcement matters into their own hands.” But DHS must proactively ensure that all ties between the agency’s Border Patrol and the militias are severed: by ordering the groups to stand down; informing them that Border Patrol will notify the FBI and state officials if they learn the groups are detaining migrants; and disciplining or removing agents who collaborate with or tacitly encourage militias.

Militias have an ugly history in the United States, particularly on the southern border, yet even before the Trump administration, the federal government did little to discourage them. Although Customs and Border Protection has acknowledged that armed vigilantes patrolling the border can have “public safety and legal consequences for all parties involved,” the recent news stories indicate that the agency’s concern has not been communicated adequately to agents on the border.

A 2016 investigative report by Mother Jones revealed a Border Patrol agent thanking a militia member by saying, “It impresses me that you guys come out and do my job for me for no pay at all,” and offering tips on where the group should patrol. A Freedom of Information Act request filed by our organization, Protect Democracy, uncovered video footage of a Border Patrol agent thanking a militia group for their efforts and asking more Americans to do the same.

Militia members tend to explain their activity by saying that they are simply supporting the Border Patrol, or defending the Constitution, or guarding the border until the wall that Trump seeks has been built, but an undercurrent of racism and potential violence is often present. The Mother Jones investigation quoted one militia member who described their mission as “hunting Mexicans,” and another who described the border as a “war zone.” The New York Times reported that responses to UCP’s videos on Facebook included “an array of antagonistic or racist descriptions of the asylum seekers.” This is not the kind of support that federal law enforcement needs or can legally allow.

The president’s irresponsible rhetoric has inflamed matters at the border, requiring urgent action by the executive branch to close off any possibility of coordinated efforts between unlawful militias and government agents. In the absence of such leadership, congressional oversight and private lawsuits will be needed.

Read more:

David Von Drehle: I’ve been to the border. Here’s what I learned.

The Post’s View: Yes, there’s a problem at the border. Trump’s wall won’t fix it.

The Post’s View: We should get more bang for our buck on border security

Tom Toles: Another global epidemic we didn’t prepare for