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ALBUQUERQUE — The F.B.I. on Saturday arrested the leader of a right-wing militia that was detaining migrant families at gunpoint near the border in southern New Mexico, as the group faced a torrent of criticism for its tactics.
Hector Balderas, New Mexico’s attorney general, said federal agents had arrested the leader, Larry Mitchell Hopkins, who had been operating under the alias Johnny Horton Jr. Mr. Balderas said in a statement that Mr. Hopkins was arrested on charges of firearms possession by a felon.
“This is a dangerous felon who should not have weapons around children and families,” Mr. Balderas said. “Today’s arrest by the F.B.I. indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not armed vigilantes.”
The firearms charge against Mr. Hopkins is relatively minor. But it is likely the start of a deeper investigation into his activities and those of the militia, and opens the way for the authorities to bring more serious charges like kidnapping and impersonating a police officer or an employee of the United States.
Mr. Hopkins’s arrest comes as tensions rise over ultraconservative paramilitary groups operating along the southwestern border. Professed militias have a long history of targeting immigrants from Latin America, tracing back to the Ku Klux Klan’s creation of its own border patrol in the 1970s. Record numbers of Central American migrants apprehended by federal authorities in recent months have been accompanied by a new surge in militia activity on the border.
The organization led by Mr. Hopkins, the United Constitutional Patriots, recently uploaded videos of armed members detaining children and their parents in a stretch of the New Mexico desert near El Paso, before handing the migrants over to the Border Patrol.
Political leaders in New Mexico, a state largely controlled by Democrats, responded with fury. Gov. Michelle Lujan Grisham said that any mistreatment of asylum seekers on the border was “unacceptable.” The state’s two Democratic senators, Tom Udall and Martin Heinrich, said in a statement that the actions of vigilante groups “cannot be tolerated.”
The American Civil Liberties Union denounced the militia’s actions in a letter on Thursday, saying the actions amounted to kidnapping by “racist and armed vigilantes.”
Still, such denunciations were far from unanimous in the state. A prominent New Mexico Republican, Gavin Clarkson, a former Trump administration official who is now running for United States Senate, met with masked members of the group in March and praised their efforts, according to a video of the encounter uploaded to Facebook.
But Mr. Clarkson said on Twitter on Saturday that he condemned militia activities. “Masked militiamen are the antithesis of what a free republic looks like,” he said.
Jim Benvie, a spokesman for the militia, did not immediately respond to requests for comment about the arrest of Mr. Hopkins.
The F.B.I. said in a statement that the police department of Sunland Park, N.M., had assisted with the arrest. Mr. Hopkins, 69, is scheduled to appear on Monday in Federal District Court in Las Cruces, N.M.
Mr. Hopkins, whose residence is in Flora Vista in northwest New Mexico, had already come under the scrutiny of groups tracking right-wing militias around the United States. He was convicted in 2006 for impersonating an officer and for felony firearm possession, according to The Daily Beast.
Despite the recent criticism over its operations in New Mexico, the militia led by Mr. Hopkins maintains a wide reach on Facebook and YouTube.
Speaking to someone using a voice distorter while wearing a gas mask, Mr. Hopkins appeared on a right-wing extremist YouTube channel in November. He claimed to be in touch with President Trump after a chance meeting at a Las Vegas casino.