ONAWAY, Mich. — The United Auto Workers union is based in Detroit, but its spiritual home lies 250 miles to the north, in a dense and remote forest on the shore of Black Lake.

Here, 40 miles from the nearest Starbucks, the U.A.W. owns a spartan retreat known to few people outside the auto industry, though it opened nearly 50 years ago. Covering 1,000 acres, the gated compound includes cabins, lodges, a banquet hall and a recreation center with an Olympic-size swimming pool. An eternal flame marks the hilltop resting place of the ashes of Walter Reuther, who built the U.A.W. into one of the most powerful unions in the country in the 1950s and 1960s.

This tranquil scene was disrupted in August when the F.B.I. raided the compound, seizing documents and records. The raid was the latest chapter in a yearslong Justice Department investigation into corruption at the union that has thrown the U.A.W. into turmoil and embroiled two of Detroit’s Big Three automakers — Fiat Chrysler and General Motors.

The resort was one of a half-dozen locations government agents searched. The raids signaled that investigators were not done with their work, despite having charged or won convictions of a dozen union officials and three Fiat Chrysler executives.