The leader of an armed militia that’s been detaining migrants at the border was just arrested over the weekend, but he’s been on the FBI’s radar since at least October 2017 for allegedly plotting assassinations, according to federal documents unsealed Monday.

The FBI received a tip in late 2017 that Larry Mitchell Hopkins, who heads the United Constitutional Patriots, claimed he was training his group to assassinate liberal billionaire George Soros, Barack Obama and Hillary Clinton because of their “support of antifa,” and was stockpiling guns despite being a felon.

The militia leader claimed he was training his group to assassinate liberal billionaire George Soros, Barack Obama and Hillary Clinton because of their “support of antifa”

But it wasn’t until antifa began circulating recent videos that showed members of the United Constitutional Patriots carrying out vigilante immigration enforcement at the New Mexico border that the feds decided to pursue charges against Hopkins. The FBI arrested the 69-year-old Sunday for being a felon in possession of a gun.

Videos posted to Facebook showed members of the militia rounding up hundreds of migrants at a time at gunpoint, including many children, forcing them to kneel in the dirt, before turning them over to Border Patrol.

Earlier this month, UCP set up a base near the border, claiming that Border Patrol had asked for their help. In a statement last week, Border Patrol said that they do “not endorse private groups or organizations taking enforcement matters into their own hands.”

In October 2017, the FBI received reports of “alleged militia extremist activity” in Flora Vista, New Mexico, just over the Colorado border. The following month, they searched 69-year old Hopkins’ residence at a Flora Vista trailer park with his permission, and “immediately observed” approximately 10 firearms leaning against a wall in plain sight. Hopkins claimed that all of the guns were owned by his wife and told investigators that she also owned two handguns and a shotgun. He also showed investigators his tactical vest, which had a loaded gun attached to it.

After searching his residence, FBI agents obtained court records that showed Hopkins had at least three prior felony convictions, including for repeated offenses involving illegal possession of loaded firearms.

In the complaint, FBI Special Agent David Gabriel says that, based on the above information they had probable cause to arrest Hopkins for being a felon in possession of a firearm.

It’s unclear why the FBI held off from arresting Hopkins, given that they had sufficient evidence to arrest him two years ago. The U.S Attorney’s Office in New Mexico did not respond to VICE News’ request for comment.

“This is a dangerous felon who should not have weapons around children and families,” New Mexico Attorney General Hector Balderas said in a statement Monday. “Today’s arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not vigilantes.”

Hopkins and the United Constitutional Patriots had been on extremism researchers’ radars for a while. A report published last November by the Southern Poverty Law Center highlighted the conspiratorial nature of the group. Members of the United Constitutional Patriots also have a radio show, according to the Daily Beast, which they use to peddle far-right conspiracy theories, including the bizarre QAnon conspiracy that claims President Donald Trump’s enemies, including Soros, Obama and Clinton, are running a child sex-trafficking ring.