A mole infiltrated the highest ranks of American militias. Here's what he found.

John Williams kept a backpack filled with everything he’d need to go on the run: three pairs of socks; a few hundred dollars cash; makeshift disguises and lock-picking gear; medical supplies, vitamins and high-calorie energy gels; and thumb drives that each held more than 100 gigabytes of encrypted documents, which he would quickly distribute if he were about to be arrested or killed.

On April 1, 2023, Williams retrieved the bag from his closet and rushed to his car. He had no time to clean...