Story highlights Reports: 1,500 works of art found in raid by German tax authorities

Paintings by Picasso, Matisse, Chagall, Klee believed to have been looted by Nazis

Experts call for works to be returned to the families of those they were stolen from

A cache of 1,500 works of art -- including masterpieces by Picasso, Matisse and Chagall -- confiscated by the Nazis and missing for more than 70 years has been found in Germany, according to German media reports.

The huge haul of paintings, estimated to be worth more than $1 billion, was discovered in an apartment in Munich in the spring of 2011 during a raid by Bavarian tax authorities, but its existence has only just come to light with an article in the German news magazine Focus

The collection is said to include works by Modernist masters Paul Klee, Oskar Kokoschka, Franz Marc, Max Beckmann, Emil Nolde and Ernst Ludwig Kirchner, many of which had been believed destroyed during World War II.

Focus reports that the pictures were found when customs police raided a rundown apartment in the Schwabing district of Munich during an investigation into suspected tax evasion. The recovered paintings are said to have been kept in storage in a secure warehouse in the city ever since.

German authorities have refused to confirm or deny any details relating to the discovery.

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"The German government is supporting the state prosecution in Augsburg by supplying advice from experts in the field of so-called degenerate art and the entire issue of Nazi-looted art," government spokesman Steffen Seibert said. "But we cannot comment any of the issues of the ongoing investigation."

The full extent of the find is expected to be revealed by art historian Meike Hoffmann, an expert on "degenerate art" from the Freie Universitaet Berlin , at a news conference in Augsburg on Tuesday.

Thousands of pieces of art labeled "degenerate" by the Nazis were looted from private collections and confiscated from galleries during the 1930s and 1940s; other works were stolen from Jewish families or sold for a fraction of their true value as the owners tried to flee the country. Many paintings remain missing decades later.

Julian Radcliffe, chairman of the Art Loss Register , said the discovery was "very encouraging in some ways, but there are tens of thousands of these artworks still under dispute, so this is really just a drop in the ocean."

"This case shows the extent of organised art robbery which occurred in museums and private collections," said Ruediger Mahlo, representative of the Conference on Jewish Material Claims against Germany . "Private collections were almost all in Jewish possession."

The relatives of many of those families whose art collections were stolen are now waiting to find out whether their property forms part of the Munich treasure trove -- and whether they can get it back.

"We demand that the paintings be handed back to their original owners," Mahlo said. "It cannot be that like in this case, a sort of moral handling of stolen goods is continued."

Radcliffe said improvements in technology and wider public knowledge about works looted by the Nazis mean the tide is turning against those in possession of stolen art.

"The risk/reward is going against them. It may have been far easier to do it decades ago, but as more databases are set up and there is an increasing focus on the Holocaust, there is less chance for anyone taking part in such transactions."

But art experts have questioned why it has taken so long for the Munich discovery to be revealed.

Christopher Marinello of Art Recovery International said the details of the paintings should have been published online as soon as they were found "so that the entire world could stake a claim."

"We were told they they were worried about the number of claimants that would come forward, but two years is a long time to wait," he said. "People died. Claimants of this era are in their 80s and 90s, and records are disappearing."

"There seems to be a problem with transparency here," Mahlo said. "The case was discovered in 2011; in 2013, it became public. The question is, what happened during those two years?"