The FBI on Saturday said it had arrested Larry Hopkins, a member of an armed group of U.S. citizens who have been stopping migrants illegally crossing the U.S.-Mexico border in New Mexico. Hopkins, 69, also known as Johnny Horton, was arrested in Sunland Park, N.M., on a federal complaint charging him with being a felon in possession of firearms and ammunition, the Federal Bureau of Investigation said in a statement. Hopkins had represented himself as the commander of the United Constitutional Patriots (UCP), a volunteer group camped out near Sunland Park since late February. The UCP has posted videos showing its members holding migrants, many of them Central American families with children seeking asylum, until U.S. Border Patrol arrives to arrest them. Migrants' surrender on videoThe group drew a complaint from the American Civil Liberties Union, and widespread media attention, after an April 16 video showed a group of around 300 migrants surrendering to the UCP at its camp. New Mexico Gov. Michelle Lujan Grisham on Friday said citizens had no authority to arrest or detain anyone and police and justice officials were looking into the UCP's activities.Hector Balderas, New Mexico's attorney general, described Hopkins as "a dangerous felon who should not have weapons around children and families." "Today's arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not armed vigilantes," Balderas said in a statement. Hopkins, of Flora Vista, N.M., is expected to have an initial appearance in U.S. District Court in Las Cruces, N.M., on Monday, the FBI said.Earlier Saturday, Mexican officials said they had "deep concern" about armed groups that intimidate and extort migrants on the border, shortly after the American Civil Liberties Union and Democratic U.S. senators called for a probe into such citizen efforts to block migrants from crossing."These types of practices can drive human rights abuses of people who migrate or request asylum or refuge in the United States," Mexico's Foreign Relations Ministry said in a statement, referring to "militia groups" in New Mexico. It added that patrols "on the margins" of the law create risks for the safety of migrants. On Thursday, the ACLU of New Mexico condemned the United Constitutional Patriots as a "fascist militia organization" operating outside the law.Dressed in camouflage The group has posted videos showing members dressed in camouflage and armed with semiautomatic rifles holding groups of migrants for pickup by Border Patrol agents.Along with the ACLU, New Mexico Democratic Sens. Martin Heinrich and Tom Udall called for an investigation into the border group. "Threatening innocent children and families fleeing violence and seeking asylum is unacceptable and flies in the face of our values as a state and a nation," they said in a joint statement on Twitter on Friday.