The FBI arrested the leader of a private militia group that detains migrants on the US-Mexico border Saturday for illegal possession of firearms and ammunition. The group operates in New Mexico.

Larry Mitchell Hopkins, who also goes by Johnny Horton Jr. according to the FBI, is a member of the United Constitutional Patriots in New Mexico. The group posted images and video of its operations on its Facebook page; some of the videos show immigrants being detained by armed members.

A recent video the group posted caught the eye of the American Civil Liberties Union (ACLU), which called on New Mexico’s governor and attorney general to both condemn the militia and end its operations.

In response to the arrest, New Mexico’s attorney general, Hector Balderas, released a statement that said in part, “This is a dangerous felon who should not have weapons around children and families.”

Balderas added, “The arrest by the FBI indicates clearly that the rule of law should be in the hand of trained law enforcement officials, not armed vigilantes.”

Mexico also chimed in on the actions of private militias on the border. Hours before news of the arrest broke, the country’s Ministry of Foreign Relations released a statement warning private militias operating on the US-Mexico border could lead to potential human rights abuses for those traveling through Mexico to request asylum from the United States.

Over a year ago, immigrant caravans traveling through Mexico captured the attention of President Donald Trump, who claimed the immigrants, many of whom hoped to request asylum, posed a danger to the United States. A number of militia groups on the border took notice as well.

A spokesperson for United Constitutional Patriots said the group has detained 5,600 immigrants in the last two months, and said the group does nothing more than “to support the Border Patrol.”

US Customs and Border Protection (CBP) has asked private citizens not to try to help it do its job. Instead, it suggests concerned citizens witnessing illegal border crossings call immigration and border officials rather than trying to apprehend immigrants themselves.