(CNN) The FBI has arrested a member of an armed militia that had detained hundreds of migrants at the border this week, the New Mexico attorney general's office said Saturday.

Larry Mitchell Hopkins, 69, is a member of an armed group that had reportedly detained migrants near Sunland Park, New Mexico, state Attorney General Hector Balderas said in a statement.

Hopkins -- also known as Johnny Horton Jr. -- was arrested on felony charges of being in possession of firearms and ammunition, according to a statement from the FBI's Albuquerque field office.

Larry Mitchell Hopkins

Earlier this week, videos posted online purported to show migrants being held by a militia known as the United Constitutional Patriots before being turned over to US Border Patrol.

The footage prompted a condemnation from the American Civil Liberties Union of New Mexico, which in a letter likened the militia's actions to kidnapping.

Read More