Speaking at the news conference, prosecutor Reinhard Nemetz said investigators have turned up "concrete evidence" that the find includes both works that the Nazis classed as "degenerate art" and seized from German museums in 1937 or shortly after, and other works that may have been taken from individuals. The Nazis often forced Jewish collectors to sell their art at pitifully low prices to German dealers or simply took them.
"Degenerate art" was largely modern or abstract works that Adolf Hitler's regime believed to be a corrupt influence on the German people. Many such works were later sold to enrich the Nazis. An art expert working with prosecutors said those sales are legally valid, even if other works in the collection may eventually be found to belong to survivors of Nazi persecution or their heirs.
The Conference on Jewish Material Claims Against Germany called for the works to be made public immediately so families of Holocaust victims could locate and recover art that had been taken from them. The conference also said that art for which heirs could not be found "must not remain with Cornelius Gurlitt, the son of a Nazi-affiliated art dealer who profited from art confiscated from Holocaust victims or sold under duress."
It said any unclaimed art should be auctioned and the money used for assistance to Holocaust survivors or Holocaust education.
Nemetz defended the delay in making the find public and rejected calls to make images immediately available on the Internet to help potential owners, citing copyright and security concerns.
Art historian Meike Hoffmann, an expert on "degenerate art" at the Free University of Berlin, offered a glimpse of some of the works during a slide show at the same news conference.
She showed works she said had not been known to scholars, or known only from documents without any photos to give an idea what the works looked like.
"Such cases are of high importance to art historians," she explained.
One Matisse painting of a woman, seized by the Nazis in France during World War II, is not in the established catalog of his works, she noted. A Chagall gouache of an allegorical scene also isn't among the artist's listed works. Experts haven't yet been able to determine where the Chagall came from, she said.
Other works, such as an unknown self-portrait by 20th-century German artist Otto Dix or a woodcut by Ernst Ludwig Kirchner, added new breadth to what's known about the artists, Hoffmann added.
Some were known works that appeared to have been legally sold, although their recent whereabouts may have been unknown. For instance, a previously listed work by Courbet of a girl with a goat made its way into the collection through an auction in 1949 — years after the end of World War II. A Franz Marc work, "Landscape with Horses," was identified as coming from an art museum in Moritzburg, Germany.
Overall, Hoffmann was elated with the quality and the depth of the find.
"When you stand in front of the works, see the ones that were long thought to have been lost or destroyed and in a relatively good state — some of them dirty but not damaged — you have an incredible feeling of happiness," she told reporters.
Frank Jordans and Geir Moulson in Berlin also contributed.