NUREMBERG, Germany — After the last of the regular artworks, furniture and stamps were sold on a recent Saturday, Kathrin Weidler read a short statement absolving Weidler’s auction house of any moral responsibility for what came next.

Ms. Weidler, who is a director of the family-run business, then began the bidding for “Village on a Mountain Lake,” a run-of-the-mill watercolor, at 45,000 euros, or about $51,000. The picture seemed indistinguishable from thousands of watercolors sold at flea markets across Europe every week, except for the signature in the bottom right corner: “A. Hitler.”

In the last 10 years, the niche market for art by Hitler has grown, experts say, and this has led to an increase in the value of the paintings, drawings and watercolors supposedly created by the future dictator over a century ago. But many, if not most, of these works are likely not by Hitler. After years of letting such public auctions go unchecked, German prosecutors are starting to take notice.

Days before the auction in February, the district attorney’s office searched Weidler’s and confiscated 63 paintings: 26 scheduled for the sale, and another 37 apparently destined for future auctions. Antje Gabriels-Gorsolke, one of the prosecutors, said the art works were seized as part of an investigation into forgery and fraud.