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 shores of Black Lake.

Here, 40 miles from the nearest Starbucks, the UAW 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 UAW into one of the most powerful unions in the country in the 1950s and 1960s.

This tranquil scene was disrupted in August when the FBI 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 UAW 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.

Last week, UAW President Gary Jones resigned as the union's executive board was preparing to remove him for the submission of false expense reports and using union money for personal gain. The UAW said Jones had concealed the use of more than $1 million in union funds for luxury travel, extravagant dinners and purchases of high-price cigars, golf clubs and apparel for himself, his family and his lieutenants.

What drew federal agents to Black Lake is not clear. But union members and labor experts have criticized the UAW's unusual decision to build a luxury lakeside cabin at the resort for the exclusive use of Jones' predecessor as president, Dennis Williams. Federal agents searched the cabin and separately raided a home owned by Williams in Corona, California. The union is reviewing other financial transactions for possible wrongdoing, and now plans to sell the cabin and the land that it sits on in a secluded corner of the retreat, said Brian Rothenberg, a UAW spokesman.

The decision to sell the cabin was part of a set of reforms instituted by the acting president, Rory Gamble, after he took the helm from Jones.

"Restoring the full faith and trust of our membership and protecting their interests is the top priority," Rothenberg said.

The UAW provided more modest, rustic cabins within the retreat complex for other previous retired presidents, and the union is considering ending that practice, too.

Union members and supporters say it is particularly disappointing that the resort — formally called the Walter P. and May Reuther Family Education Center — has been pulled into the investigation. The union uses it as a place for members to study the past and plan for the future in training seminars and conferences.

"Black Lake serves a critical purpose for the union," said Harley Shaiken, a labor relations professor at the University of California, Berkeley, who has attended and taught seminars at the retreat since the 1970s. "It's historically important for the union. It's meant to get people together to debate critical issues. It's a place of thinking and engaging."

When the retreat opened in 1970, it was heralded for its timbered beams and spare Scandinavian design.

Today, rather than a showplace for the UAW, Black Lake is a part of the union's financial difficulties. The union spends several million dollars a year to operate it — money that is classified as a loan to the subsidiary that operates Black Lake. In 2018, the accumulated debt amounted to more than $60 million.

Anyone can book a stay there. During a recent overnight stay, I found it dated. Guest rooms feature few of the amenities found in modern hotels, like Wi-Fi and cable TV. Some hallways were marked by musty odors. But a golf course the UAW built just outside the retreat in 2000, at a cost of $6.7 million, is considered one of the best in Michigan.

These days, senior union officials often hold meetings in places like Orlando, Florida, Washington, D.C., and May, New Jersey. Before becoming the UAW president in 2018, Jones ran a regional office in Hazelwood, Missouri, and held annual conferences in Palm Springs, California. Court filings by federal prosecutors and an internal union complaint against Jones claim that he and other union officials booked luxury villas for four weeks or more, even though the Palm Springs conferences lasted less than a week.

In raids at Jones' home and elsewhere, agents seized thousands of dollars in cash, hundreds of bottles of high-price liquor, hundreds of golf shirts, multiple sets of golf clubs and large quantities of cigars that had been billed to union accounts, according to court filings. One dinner described by prosecutors ran up a bill of $6,599.87 that included $1,760 for four bottles of Louis Roederer Cristal Champagne.

A lawyer for Jones, J. Bruce Maffeo, said the suggestion that the conference expenses had been concealed "is without any basis in fact." Spending for the conferences in Palm Springs was "laid out in sufficient detail to the UAW accounting department over a period of years."

A close associate of Jones who helped organize the conferences in Palm Springs and has been charged by federal prosecutors, Vance Pearson, resigned from the UAW on Sunday. The union had been taking steps to remove him from his post as a regional director.