Officials in southern Germany have revealed that a hoard of paintings found in Munich includes previously unknown masterpieces by modernist painters such as Henri Matisse and Marc Chagall.
As slides of the paintings were shown at a news conference, an expert said the works had been seized from private individuals or institutions.
The collection consists of 1406 artworks; modern paintings banned by the Nazis as "degenerate art" and older masters looted from mostly Jewish families in occupied lands, particularly France.
The total value has been estimated at about €1 billion.
Previously unregistered works by Marc Chagall, Otto Dix, Franz Marc, Max Liebermann and Henri Matisse were found. Other artists whose works were found include Pablo Picasso and Henri de Toulouse-Lautrec, as well as Canaletto and Gustave Courbet.
Officials said the works were seized in the Munich flat of Cornelius Gurlitt, the son of prominent Nazi-era art dealer Hildebrand Gurlitt, who acquired the paintings in the 1930s and 1940s.
The paintings were found in March last year, not, as reported, in 2011, after Mr Gurlitt was investigated for tax evasion.
Tax and customs investigators said they had questioned Mr Gurlitt but no longer knew his whereabouts.
The investigators said it had taken so long to make the matter public because of the complexity of identifying ownership.
The BBC reports that descendants of owners of art taken by German forces in occupied Europe may be able to make claims.
However, there are suggestions that art taken in Germany itself may not be reclaimable because of a 30-year statute of limitations.