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The painting was given away during a retirement party. Years later, a descendant of the recipient tried to sell the art. An auction house refused to part with the work until the Justice Department filed papers to freeze the title.

“They come up in all different ways,” said Mr. Miller. “Some of them have been in a neighbor or a grandmother’s attic. They all have stories.”

Mr. Miller said the GSA inspector general’s office keeps an eye on the online auction site eBay, where agents have spotted and recovered paintings such as “German Restaurant” by Antoinette K. Gruppe and “The Accident” by Andrian Troy, a 1936 painting valued at about $25,000.

Sometimes, people who know they have government-owned art try to offload it and cash in before they are caught. After GSA inspector general agents contacted someone trying to sell “The Accident” on eBay, the seller, who later fled to Europe, stripped the Works Progress Administration stamp from the painting and sold it for a fraction of its value to an auction house, which ultimately returned the piece.

Serving nearly a decade as the federal bureaucracy’s inspector general, Mr. Miller frequently appeared on Capitol Hill to expose waste and fraud at the GSA, including a series of high-profile cases involving lavish conference expenditures.

He said the art recovery project gave his agents a chance to work with GSA officials in a more collaborative way.

“We can do something that directly helps them do their jobs and we get to put out art that belongs to the public for them to see,” he said.