The FBI on Saturday arrested a man described as a commander of an armed group that has been detaining migrants in New Mexico, the state attorney general’s office said.

Larry Mitchell Hopkins, 69, of Flora Vista, New Mexico, was arrested for allegedly being a felon in possession of firearms and ammunition, New Mexico Attorney General Hector Balderas and the FBI said.

The arrest comes after New Mexico Gov. Michelle Lujan Grisham, a Democrat, demanded that members of a militia group, some of whom are armed, stop detaining migrants at the New Mexico-Mexico border.

Larry Mitchell Hopkins Dona Ana County Detention Center

Balderas said in a statement that Hopkins is "a member of the armed group of individuals reportedly detaining migrants near Sunland Park, New Mexico."

"This is a dangerous felon who should not have weapons around children and families," Balderas said. "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."

Jim Benvie, a spokesman for United Constitutional Patriots, said Hopkins is their "national commander."

In a letter Thursday to Lujan Grisham and Balderas, the American Civil Liberties Union of New Mexico said United Constitutional Patriots had arrested nearly 300 people Tuesday near Sunland Park, a city about 10 miles from El Paso, Texas.

Hopkins is scheduled to appear in federal court Monday in Las Cruces. It was unclear if he has an attorney. A message left at a phone number that public records show might belong to Hopkins in Flora Vista was not immediately returned Saturday.

The FBI said no other information would be released until after Hopkins' court appearance, and it was not immediately clear what the underlying felony conviction was.

Striker, the leader of the Constitutional Patriots militia, speaks with Viper, right, about logistics in Anapra, New Mexico, on March 20, 2019. Members go by aliases to protect their identities. Paul Ratje / AFP / Getty Images file

Lujan Grisham said this week that state officials would be working with local police to stop the militia group from arresting migrants.

Balderas warned that "these individuals should not attempt to exercise authority reserved for law enforcement."

Benvie claimed Saturday that Hopkins' arrest was politically motivated and that the United Constitutional Patriots is an "armed patriot group" not an armed militia. He said Hopkins is a Vietnam veteran and former "fugitive recovery officer," or bounty hunter.

"I believe that this was an attack from the governor ordered to the AG's office to come out to our camp and find something that they could find to either shut us down, get us removed from the property that we’re on, or do something else to harm the integrity of the organization," Benvie said.

The FBI said it arrested Hopkins on a federal complaint and did not mention an armed group.

Reuters reported Thursday that United Constitutional Patriots, made up mainly of veterans, has been patrolling in the Sunland Park area since late February. They have been posting videos almost daily of members in camouflage, armed with semi-automatic rifles, holding asylum-seekers until Border Patrol officers arrive, the news agency reported.

Members of the United Constitutional Patriots share cigarettes while on patrol in Sunland Park, New Mexico, on March 20, 2019. Paul Ratje / AFP - Getty Images file

U.S. Customs and Border Protection said in a statement that it “does not endorse private groups or organizations taking enforcement matters into their own hands.”

Armed civilian groups have been a fixture on the border for years, especially when large numbers of migrants come, the Associated Press reported. 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 AP reported.