Anne Webber, a founder of the commission, said her researchers concluded that the resale of looted art to Nazi-tied families had hardly been isolated. “They called them a ‘return sale,’” she said. “Why were they returned to them rather than the family from whom they were looted? Nobody knew.”

The return sales to Nazi families — first reported in late June by the Munich daily Süddeutsche Zeitung — are also causing political recriminations. This week, a Bavarian state Parliament committee demanded an accounting from government officials about the extent of the system to resell art to Nazi families and a tally of how many looted works remain in government possession that could be returned to the proper heirs.

Mrs. von Schirach’s grandson Ferdinand von Schirach — a best-selling German author whose grandfather was imprisoned for 20 years for the deportation of more than 60,000 Austrian Jews — is also pledging to investigate the provenance of his late grandmother’s art.

“We need to know about the evil,” he said. “That’s the only way we can live with it.”

The discreet art trade with Nazi relatives emerged in 1949, four years after the war, when the American military transferred responsibility for restitution of looted works to the West Germans and Austrians. Munich became the hub of a network of art dealers and state officials who had helped drive Nazi looting and then after the war fostered trafficking in those works.

The painting collection’s archives illustrate in great detail the efforts by families like the von Schirachs to retain the art. The family patriarch was Baldur von Schirach, a Nazi governor in Vienna who was tried for war crimes in Nuremberg. While he was imprisoned in Spandau, his wife, Henriette, and relatives worked for more than a decade to reclaim art, carpets and furniture.

Image The small oil painting “View of a Dutch Square,” apparently from the studio of the Dutch artist Jan van der Heyden in the late 17th century. Credit... Kraus Family and Commission for Looted Art in Europe

According to archival documents, her father, Heinrich Hoffmann, who was Hitler’s personal photographer, sought to claim a list of 278 paintings that had been confiscated by units like the Monuments Men, an Allied group devoted to recovering and returning looted art.