FARMINGTON, N.M. — The FBI arrested a New Mexico man Saturday on charges of being a felon in possession of a firearm and ammo as controversy continues over his group’s deployment of armed volunteer border patrollers who have detained undocumented immigrants crossing into the U.S.

An FBI release said agents and Sunland Park police arrested Larry Mitchell Hopkins, 69, of Flora Vista, who is also known as Johnny Horton Jr., “on a federal complaint charging him with being a felon in possession of firearms and ammunition.”

Shortly before 4 p.m. the United Constitutional Patriots declared an "emergency" on their Facebook page, stating that their commander had been arrested at their camp near the Mexican border, and asking for donations to a fundraising account.

Doña Ana County records indicate Hopkins was arrested at 11:55 a.m. on the federal warrant and is being held without bond. Hopkins is expected to appear in the U.S. District Court in Las Cruces on Monday morning.

More:Armed civilians are detaining migrants at the border. The ACLU wants them investigated

Hopkins was with the United Constitutional Patriots, a group that drew the ire of the American Civil Liberties Union and New Mexico Attorney General Hector Balderas after it began detaining people they encounter who crossed into U.S. territory.

“This is a dangerous felon who should not have weapons around children and families,” Balderas said Saturday in a statement. “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.”

The FBI declined comment about Hopkins' role in the group, stating more public information will be available Monday after Hopkins appears in court. Hopkins identified himself as the national commander of the militia group in an interview last month with El Paso ABC television affiliate KVIA.

The Facebook page for United Constitutional Patriots, linked to by Hopkins on his Facebook page, states that United Constitutional Patriots is for “Americans that believe in the constitution and the rights of every American that will stand up for there (sic) rights in unity and help keep America safe.”

The Associated Press reported that U.S. authorities 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 over the last several days 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 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," the agency said on its Twitter account.

Jim Benvie, a spokesman for the group, said members assist a "stressed and overstrained Border Patrol" and there are no issues with its actions. One video shows a Border Patrol agent arriving after the group tells a small group to sit and wait.

New Mexico Gov. Michelle Lujan Grisham, a Democrat, said Friday that she was actively working with state and local police to address the armed groups. "Menacing or threatening migrant families and asylum-seekers is absolutely unacceptable and must cease," she wrote on Twitter.

The American Civil Liberties Union of New Mexico on Thursday wrote Lujan Grisham and state Attorney General Hector Balderas on Thursday, urging them to investigate the incidents, including what it said was the detention of nearly 300 people on Tuesday near Sunland Park.

"This has no place in our state: we cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum," the ACLU wrote.

Contributing: The Associated Press