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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.”