The artwork, originally either confiscated as "degenerate art" by the Nazis or taken from Jewish collectors in the 1930s and 1940s, wound up with a German art collector, Hildebrand Gurlitt, who passed them to his son when he died, without the knowledge of the authorities, officials said.
The younger Gurlitt attracted attention of customs authorities after a random cash check during a train trip from Switzerland to Munich in 2010, the German Focus magazine reported. Police investigations led to a raid on Gurlitt's apartment in the spring of 2011, then they discovered the masterpieces.
The Guardian said the works were initially believed to have been stored food on homemade shelves. Since their seizure, they have been stored in a customs building near Munich, where the art historian Meike Hoffmann has been assessing their origin and value.
At least 300 paintings are thought to belong to a group of about 16,000 works declared "degenerate art," a term the Nazi regime used to describe nearly all modern art, The Guardian said. Others are suspected to have been owned by Jewish collectors who were forced to leave their belongings behind when they fled.
Gwendolen Webster, an art historian who studied works from the Nazis' degenerate art collection, told The Guardian the significance of the find was "absolutely staggering for historians" but opened a legal can of worms because of the number of claims likely to be filed.