The head of an armed group stopping migrants who cross the United States-Mexico border illegally allegedly boasted of training volunteers to kill former President Barack Obama and former Secretary of State Hillary Clinton, an FBI agent says.
Larry Hopkins, leader of the United Constitutional Patriots (UCP), appeared in court in Las Cruces, New Mexico, on Monday to face charges of being a felon in possession of a firearm.
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The FBI said it found nine guns during a 2017 visit to his home.
The UCP has helped the US Border Patrol detain some 5600 migrants in New Mexico in the last 60 days, the group said.
Defence lawyer Kelly O'Connell said Hopkins planned to plead not guilty to the firearms charge.
"This is not even dealing with what's going on right here," O'Connell said.
The UCP has said its two-month presence at the border was intended to support US Border Patrol, which has been overwhelmed by record numbers of Central American families seeking asylum.
In court papers, the FBI said witnesses in 2017 accused Hopkins of saying the UCP was training to assassinate Obama; Clinton, who was the Democratic presidential candidate in 2016; and George Soros, a financier who supports liberal causes.
The accusations were made during an investigation into "militia extremist activity," the agency said.
The American Civil Liberties Union and other critics accuse the UCP of being a "fascist militia" whose members illegally detain and kidnap migrants by impersonating law enforcement.
UCP members, many of whom have served with US Special Forces, take turns living in a camping trailer close to the border near Sunland Park, New Mexico and patrolling an 8km section of border, much of it unfenced.
Armed with rifles and wearing camouflage uniforms with the group's eagle insignia, the group has posted dozens of videos showing the volunteers instructing migrant families to sit and wait until Border Patrol agents arrive.
The group has been accused of detaining women and children at gunpoint, a claim it fiercely denies.
New Mexico's Democratic Governor, Michelle Lujan Grisham, on Friday ordered an investigation of the UCP.
She said "menacing or threatening migrant families and asylum seekers is absolutely unacceptable and must cease."
FBI Special Agent David Gabriel said that in October 2017 the agency received reports a militia was being run out of Hopkins' home in Flora Vista, New Mexico.
When agents entered the home they collected nine firearms, ranging from pistols to rifles, Horton was illegally in possession of as he had at least one prior felony conviction, according to the complaint.
Hopkins, the UCP's national commander, told the agents his common-law wife owned the weapons.