The FBI on Saturday said it had arrested Larry Hopkins, the leader of an armed group that is stopping undocumented migrants after they cross the U.S.-Mexico border into New Mexico.

The arrest came two days after the American Civil Liberties Union (ACLU) accused the group of illegally detaining migrants and New Mexico¡¯s Democratic Gov. Michelle Lujan Grisham ordered an investigation.

Hopkins, 69, also known as Johnny Horton, was arrested in Sunland Park, New Mexico, on a federal complaint charging him with being a felon in possession of firearms and ammunition, the Federal Bureau of Investigation said in a statement.