After more than two years undercover, he’d been growing rash and impulsive. He had feared someone was in danger and tried to warn him, but it backfired. Williams was sure at least one person knew he was a double agent now, he said into his phone. “It’s only a matter of time before it gets back to the rest.”

In the daylight, Williams dropped an envelope with no return address in a U.S. Postal Service mailbox. He’d loaded it with a flash drive and a gold Oath Keepers medallion.

It was addressed to me.

The documents laid out a remarkable odyssey. Posing as an ideological compatriot, Williams had penetrated the top ranks of two of the most prominent right-wing militias in the country. He’d slept in the home of the man who claims to be the new head of the Oath Keepers, rifling through his files in the middle of the night. He’d devised elaborate ruses to gather evidence of militias’ ties to high-ranking law enforcement officials. He’d uncovered secret operations like the surveillance of a young journalist, then improvised ways to sabotage the militants’ schemes. In one group, his ploys were so successful that he became the militia’s top commander in the state of Utah.