The FBI said in a statement it arrested 69-year-old Larry Mitchell Hopkins in Sunland Park with the assistance of local police. New Mexico Attorney General Hector Balderas said in a separate statement that Hopkins was a member of the group that had stopped migrants.
Hopkins was booked into the Dona Ana County detention center in Las Cruces and it wasn't immediately known whether he has an attorney who could comment on the allegations.
The FBI statement did not provide information on Hopkins' background, and FBI spokesman Frank Fisher told The Associated Press that no additional information would be released until after Hopkins has an initial appearance Monday in federal court in Las Cruces.
The FBI said Hopkins is from Flora Vista, a rural community in northern New Mexico and approximately 353 miles (572 kilometers) north of Sunland Park, which is a suburb of El Paso, Texas.
The Sunland Park Police Department on Saturday referred an AP reporter to the FBI.
Balderas said in a statement that Hopkins "is 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."
Federal authorities on Friday warned private groups to avoid policing the border after a string of videos on social media showed armed civilians detaining large groups of Central American families in New Mexico.
The videos posted earlier in the week show members of United Constitutional Patriots ordering family groups as small as seven and as large as several hundred to sit on the dirt with their children, some toddlers, waiting until Border Patrol agents arrive.
Customs and Border Protection said on its Twitter account that it "does not endorse or condone private groups or organizations that take enforcement matters into their own hands. Interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved."
Jim Benvie, a spokesman for United Constitutional Patriots, did not immediately respond Saturday to a request for comment made via Facebook.
Benvie said in a video that the group's members were assisting a "stressed and overstrained Border Patrol" and said the group is legally armed for self-defense and never points guns at migrants. The posted videos do not show them with firearms drawn.
Armed civilian groups have been a fixture on the border for years, especially when large numbers of migrants come. But, unlike previous times, many of the migrants crossing now are children.
In the Border Patrol's El Paso sector, which has emerged as the second-busiest corridor for illegal crossings after Texas' Rio Grande Valley, 86% of arrests in March were people who came as families or unaccompanied children.
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