The arrest comes after the American Civil Liberties Union (ACLU) branch of New Mexico warned on Thursday the UCP were illegally detaining migrants.

The U.S. Federal Bureau of Investigation (FBI) said on Saturday it arrested Larry Hopkins, 69, the leader of far-right armed militias that were illegally stopping undocumented migrants after they crossed the U.S.-Mexico border into New Mexico.

“The detention by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not armed vigilantes,” New Mexico Attorney General Hector Balderas said in a statement.

Also known as Johnny Horton, the perpetrator is the “national commander” of the far-right United Constitutional Patriots (UCP) militia. He was detained on a federal complaint charging him with being a felon in possession of firearms and ammunition, the FBI said in a statement.

“We’re not worried about it, he’s going to be cleared,” said Jim Benvie, a spokesman for the UCP, blaming his arrest on political pressure from New Mexico’s governor. In 2006, Hopkins was previously arrested in Oregon on suspicion of impersonating a police officer and being a felon in possession of a firearm, according to the Southern Poverty Law Center.

U.S. Customs and Border Protection (CBP) said the agency did not need the help of citizens to police the border. In the picture two CBP officers posing with a militia member. Photo: UCP Facebook

Yet this new arrest comes after the American Civil Liberties Union (ACLU) branch of New Mexico warned on Thursday the UCP were illegally detaining migrants, approximately 300 people, mainly from Central America seeking asylum. The Mexican Ministry of Foreign Affairs expressed concern about the activities of intimidation and extortion of migrants by illegal armed groups.

The ACLU has described them as a "fascist militia organization" and that the group is a product of the Trump administration’s “vile racism” that “has emboldened white nationalists and fascists to flagrantly violate the law.” It is mainly made up of military veterans.

UCP members have been patrolling the U.S.-Mexico border near Sunland Park, New Mexico, since late February in search of undocumented border crossers. They post near-daily videos showing members dressed in camouflage and armed with semi-automatic rifles holding groups of migrants until U.S. Border Patrol agents arrive.