Transcript for Leader of Oath Keepers militia group arrested in connection to the Jan. 6 riot
REPORTER: A major escalation in the Justice Department's January 6 investigation, leveling its most serious charge yet. Tonight, Stewart Rhodes, founder and leader of the extremist militia group the Oath Keepers, is behind bars. He and 10 other alleged group members charged with seditious conspiracy-- the first time that accusation has been connected to January 6.
MIKE LEVINE: Sedition is such a big thing here because the government is alleging that force was used to basically overthrow the government or to usurp the government's authority.
- This isn't just charges of trespasses. Like, OK, people got a little caught up in the moment. They trespassed. Premeditating your plan to overthrow the US government, it's a form of treason. It is a form of terrorism.
REPORTER: In a newly unsealed indictment, they're accused of conspiring to oppose by force the execution of the laws governing the transfer of presidential power from former President Trump to President Joe Biden. The indictment accusing the defendants of organizing into teams, prepared and willing to use force and to transport firearms and ammunition into Washington DC, of recruiting members to participate organizing trainings in paramilitary combat tactics and bringing supplies, including knives, batons, camouflage combat uniforms, tactical vests with plates, helmets, eye protection, and radio equipment to the Capitol grounds.
Rhodes has been under investigation since last spring. If found guilty of seditious conspiracy, he could serve a maximum of 20 years in prison.
- It sends a signal to anybody that thinks that they're not happy with the way the government is working. And so I'm going to use violent means to address my grievance. It says they will find you, and you will be held to account for that.
REPORTER: The FBI'S case built on accessing encrypted apps that the group allegedly used to secretly communicate with one another. According to the indictment, two days after the election, Rhodes allegedly messaged followers-- "we aren't getting through this without a Civil War. Too late for that. Prepare your mind, body, spirit." And days later writing, "we must now do what the people of Serbia did when Milosevic Stole their election-- refuse to accept it and March unmasked on the nation's capital--" doubling down publicly on conspiracy theorist Alex Jones radio show.
- We have men already stationed outside DC as a nuclear option. In case they attempt to remove the president illegally, we will step in and stop it.
REPORTER: All this reaching a fever pitch when then-president Trump held a Save America rally blocks from the Capitol, demanding the election results be overturned.
- We're going to walk down any one you want, but I think right here we're going to walk down to the Capitol.
You'll never take back our country with weakness. You have to show strength and you have to be strong.
REPORTER: Demonstrators soon climbing the walls and scaffolding, violently confronting law enforcement.
REPORTER: It was clear that the crowd was intent on causing harm to our officers.
- They're breaking the window.
REPORTER: As the mob moved in, the entire Capitol went into lockdown.
Law enforcement was no match for the sheer size of the invasion-- officers forced to retreat. The indictment including alleged encrypted messages between Oath Keepers from that afternoon sent on the app Signal, saying that Rhodes messaged, "Pence is doing nothing. As I predicted." Adding, "All I see Trump doing is complaining. I see no intent by him to do anything." So the patriots are taking it into their own hands. They've had enough."
The indictment also claims Rhodes stayed outside the Capitol building while other members breached the rotunda doors inside. And they later headed towards the House of Representatives in search of Speaker Pelosi before linking back up with Rhodes and others outside.
Oath Keepers reportedly claimed to have been at the Capitol as part of a security detail and Rhodes' attorney says the government's case is based on lies. Rhodes previously told the "New York Times" that he "did express frustration that some of my guys went in. That those who breach the Capitol had gone off mission and that there was zero instructions from me or leadership to do so."
- The Oath Keepers and a lot of these anti-government militia groups, they are careful not to call for violence. What they have said for the past decade, for longer than that, is that people's rights are in danger, and they would have to stand up to sort of save their rights and protect America.
- He has moved more from a place of, we are going to be the protectors of our community in case of a bad day to the bad day is here, the Civil War is happening. It is time to go and defend ourselves from the infringements that the federal government has made in our lives.
REPORTER: 56-year-old Stewart Rhodes founded the Oath Keepers in 2009, a far-right anti-government militia group that focused on recruiting ex-military, former law enforcement officers, and first responders.
- He was in the army for a little bit and then left the army, and went to Yale Law School.
- Ended up working for the former Representative Ron Paul.
- He lost his eye when he shot it out himself with a Derringer. He's a type of person who read six books at once, but at the same time, there's a lot of common sense that he seems to be lacking.
REPORTER: Colorado artist and writer Jason Van Tatinhove spent years working with Rhodes for the Oath Keepers.
- You saw Stewart Rhodes up close and personal what makes you so convinced that he's guilty of these charges?
- Well, you know, Stewart lived in my basement for several months towards the end of my tenure with the Oath Keepers. And we spent a lot of time together, you know? We got to know each other on a pretty personal level.
He spoke of things like this-- what he would call a quick reaction force nearby, you know, with better, more heavily armed. This is something that he's been doing as a leader of the Oath Keepers for quite a while.
- You saw the inner workings of the Oath Keepers. How dangerous was the organization?
- I think the biggest threat comes in their ideas, comes in their messaging. The real danger comes from kind of the lone wolf extremist set that are on the peripheral that are listening Alex Jones, that are listening to Stewart Rhodes, you know, and taking it to heart.
Hey, guys, Jason Van Tatenhove of Oath Keepers here.
REPORTER: Jason was featured in the ABC News documentary "Homegrown, Standoff to Rebellion."
- I just got to own it. You know what? I was swept up I was excited. And I was wrong. And I do-- I feel awful about it. I think I needed a wider pulled-out view to understand things better.
REPORTER: The ex-Oath Keepers spokesperson left the group in 2018, now rejecting his old rhetoric as dangerous propaganda.
JASON VAN TATENHOVE: I was an independent journalist at the time, and he actually reached out with a job offer.
- Did you feel as though you were working for the Oath Keepers, or did you feel like you were a member of the Oath Keepers?
- Oh, no, I was definitely always working for the Oath Keepers, but, you know, the line's kind of blurred. And I became, in time, a propagandist for the Oath Keepers. I started noticing, you know, that a lot of my stories were being heavily edited or just outright refused. And you know, that should have been my first warning sign. I should have probably just walked away then.
- What was your reaction to the charges of seditious conspiracy?
- It was bittersweet. There was a sense of relief, but, you know, trepidation as well because we've seen every time so far that these people are not held to account. So I have hope that with the largest case ever brought forward by the Department of Justice that we may see some real tangible results. But you know, I'm not holding my breath.
- Rhodes is set to appear in court tomorrow.
This transcript has been automatically generated and may not be 100% accurate.