A member of an armed rightwing militia group accused of illegally detaining migrants at the US-Mexico border has been arrested, officials said on Saturday.

Videos appear to show armed militia detaining migrants at US-Mexico border Read more

The FBI arrested Larry Mitchell Hopkins, 69, for alleged unlawful possession of firearms and ammunition , days after his group posted videos that appeared to show armed men stopping migrants at the border in New Mexico, ordering them to sit on the ground and coordinating with US border patrol agents to have them taken into custody.

“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 New Mexico attorney general, Hector Balderas, said in a statement.

On Thursday, the American Civil Liberties Union (ACLU) called for an investigation into the pro-Trump, anti-immigrant men who have been patrolling the border and calling themselves the United Constitutional Patriots (UCP).

The ACLU in New Mexico described the group as “an armed fascist militia organization” made up of “vigilantes”, saying they were working to “kidnap and detain people seeking asylum” and had directly made illegal arrests and held migrants at gunpoint.

Hopkins’ role in the group was unclear, and it was also not clear if he was facing any charges directly related to the videos of the men stopping migrants. The New Mexico attorney general’s office described Hopkins as a “dangerous felon who should not have weapons around children and families” and labeled him an “armed individual detaining migrants at the border”.

An FBI statement on the arrest did not mention the militia group or the detention of migrants, but noted that Hopkins has also gone by the name Johnny Horton Jr and that he was due to appear in court on Monday.

In one video posted last Monday night by Jim Benvie, a member of the armed group, it appeared the men were ordering around dozens of migrants, including many children, and telling them to sit. Benvie narrated: “There’s no border patrol here. This is us.”

The men appeared to call border patrol agents, who later were seen. The ACLU, in a letter to state officials, said the group had targeted nearly 300 migrants in Sunland Park, New Mexico, which is along the Mexico border and adjacent to El Paso, Texas.

The militia members advocated for Trump’s proposed border wall on the video streams and echoed the president’s anti-migrant rhetoric, warning of an “invasion”.

There have also been concerns that the men, who wear military-style clothes, could be misrepresenting themselves as border patrol agents. In another video, Benvie filmed himself stopping a group of four adults and three children and said “border patrol” as he approached, before calling for another member of his group to join him. In an apparent call to border patrol, one man said: “Hello, I’ve got seven over here.”

The UCP has previously presented itself as a group of “volunteers” aiding border patrol and supporting Trump. In recent years, there have been numerous reports of an increase in paramilitary groups and xenophobic activists surveilling the border, working with US agents and targeting undocumented people.

The UCP has not responded to the Guardian’s requests for comment.

On Thursday, a Customs and Border Protection spokesperson said border patrol “welcomes assistance from the community” but “does not endorse private groups or organizations taking enforcement matters into their own hands”.

The Daily Beast reported that Hopkins has a history of pushing far-right conspiracies and has a conviction for impersonating an officer and felony firearm possession. He could not be reached on Saturday and it was not immediately clear if he had a lawyer.

Stephanie Corte, an immigrant rights campaign strategist with the ACLU in New Mexico, said her group was still trying to make contact with the detained migrants and was uncertain if any had been released.

“Our next focus is to try to get their story and making sure they feel safe to tell their story of being held at gunpoint,” she told the Guardian, adding that the ACLU would explore legal options.

She said she hoped authorities would work to make sure this does not happen again.

“We absolutely believe that this was completely unlawful,” she said. “We’re hoping those involved will be brought to justice.”

