New Mexico Attorney General Hector Balderas said in a statement that United Constitutional Patriots member Larry Mitchell Hopkins, 69, is a "dangerous felon who should not have weapons around children and families."
A United Constitutional Patriots spokesperson attacked Balderas and New Mexico Gov. Michelle Lujan Grisham for the arrest.
"I am confident that Mr. Hopkins will get through this, will fare well," the militia's spokesperson, Jim Benvie, told El Paso ABC affiliate KVIA. "The [New Mexico] AG has declared war on American citizens at the order of the ACLU, instructing the governor, in a sense, to effectively find a reason to remove private citizens from assisting and documenting a crisis on the border. It's really sad that she can't use the resources of the National Guard or even the FBI, if they had to, to help protect the border. Instead, they had to infiltrate and set up our camp, and we're confident about our position with this.
"We're not worried about it. It doesn't change anything," he added.
Hopkins was convicted of impersonating an officer and felony gun possession in 2006, according to The Daily Beast.
Speaking to KVIA earlier this year, Hopkins said, "This country was built on three things: God, guns and guts. That American flag has to keep flying. It's in danger, a lot of danger right now."
ORIGINAL REPORT: Border Patrol pushes back against armed civilians policing border
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."
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.
The Associated Press and ABC News contributed to this report.