• A Freelance Vigilante: A wilderness survival trainer spent years undercover, climbing the ranks of right-wing militias. He didn’t tell police or the FBI. He didn’t tell his family or friends.
• The Future of Militias: He penetrated a new generation of militia leaders, which included doctors and government attorneys. Experts say that militias could have a renaissance under Donald Trump.
• A Secret Trove: He sent ProPublica a massive trove of documents. The conversations that he secretly recorded give a unique, startling window into the militia movement.

So I pored over his files, tens of thousands of them. They included dozens of hours of conversations he secretly recorded and years of private militia chat logs and videos. I was able to authenticate those through other sources, in and out of the movement. I also talked to dozens of people, from Williams’ friends to other members of his militias. I dug into his tumultuous past and discovered records online he hadn’t pointed me to that supported his account.

The files give a unique window, at once expansive and intimate, into one of the most consequential and volatile social movements of our time. Williams penetrated a new generation of paramilitary leaders, which included doctors, career cops and government attorneys. Sometimes they were frightening, sometimes bumbling, always heavily armed. It was a world where a man would propose assassinating politicians, only to spark a debate about logistics.

Federal prosecutors have convicted more than 1,000 people for their role in Jan. 6. Key militia captains were sent to prison for a decade or more. But that did not quash the allure that militias hold for a broad swath of Americans.

Now President-elect Donald Trump has promised to pardon Jan. 6 rioters when he returns to the White House. Experts warn that such a move could trigger a renaissance for militant extremists, sending them an unprecedented message of protection and support — and making it all the more urgent to understand them.

[Williams] took note of the job titles of leaders he met, like an Air Force reserve master sergeant (I confirmed this through military records) who recruited other airmen into the movement. Williams attended paramilitary trainings, where the group practiced ambushes with improvised explosives and semiautomatic guns. He offered his comrades free lessons in hand-to-hand combat and bonded with them in the backcountry hunting jackrabbits. When the militia joined right-wing rallies for causes like gun rights, they went in tactical gear. Williams attended as their “gray man,” he said — assigned to blend in with the crowd and call in armed reinforcements if tensions erupted.

Since his work was seasonal, Williams could spend as much as 40 hours a week on militia activities. One of his duties as intel officer was to monitor the group’s enemies on the left, which could induce vertigo. A militia leader once dispatched him to a Democratic Socialists of America meeting at a local library, he said, where he saw a Proud Boy he recognized from a joint militia training. Was this a closet right-winger keeping tabs on the socialists? Or a closet leftist who might dox him or inform the police?