“That’s the way that restitution works,” Mr. Petropoulos added, calling it “unconscionable” that the authorities “sat on the trove for two and a half years,” particularly because it appeared to be an exceptionally large find.

The trail to the artworks, the magazine said, stemmed from an incident in September 2010, when Bavarian customs officials on a train to Germany from Switzerland became suspicious after finding Mr. Gurlitt carrying €9,000, or about $12,150, in crisp €500 notes.

The inquiries spurred by the money eventually led investigators to the apartment in Munich, the magazine said, reporting that Mr. Gurlitt had apparently lived there for decades, selling off pictures as needed over the years, to judge by empty frames found in his home. Emma Bahlmann, an employee of the Cologne auction house that sold the Beckmann work, said she went to an apartment with Mr. Gurlitt but saw no evidence of other artworks as she took the Beckmann off the wall.

The hundreds of works found in the Munich apartment reported to have been raided by authorities — including paintings but also many graphics and even an engraving by Albrecht Dürer, the German Renaissance artist — were taken to a customs facility near Munich for storage, Focus said. Meike Hoffmann, an art historian at an institute specializing in Nazi-confiscated art at the Free University in Berlin, was engaged to go through the discovered works.

Ms. Hoffmann declined to talk to reporters on Sunday or Monday about what she described in an email as “this case.”

But a video of a conference in September, posted on the institute’s website, showed her saying that her institute would soon be doing more work associated with Hildebrand Gurlitt, Cornelius Gurlitt’s father. The elder Mr. Gurlitt had trouble with the Nazis because he was deemed a quarter Jewish under the Nuremberg race laws, and he was dismissed from two museum posts. Yet he was also one of the few Germans granted permission by Joseph Goebbels, Hitler’s propaganda chief, to sell confiscated art. Sales to foreign buyers were meant to fill Nazi coffers, but art historians have documented many sales in Germany, as well as proceeds pocketed by the dealers involved.