The discovery in a rubbish-strewn flat in Germany of nearly 1,500 paintings including works by Picasso and Matisse looted by the Nazis sparked urgent calls Monday to hunt for their rightful owners.
The shock find was reported Sunday by news weekly Focus, which valued the works at around one billion euros ($1.3 billion dollars). Authorities repeatedly declined to comment on the trove uncovered in 2011 but scheduled a news conference on Tuesday.
However German government spokesman Steffen Seibert said Berlin had been aware of the case for "several months" and was assisting an investigation by public prosecutors with experts in Nazi-era stolen art. He said he was unaware of any restitution demands.
Hundreds of the modernist masterpieces are believed to have been stolen by the Nazis from Jewish collectors or seized as part of crackdowns on "degenerate art".
"I think it's the biggest single find of Holocaust pictures that there's been for years, but it's still a tiny fraction of the total number of pictures that we're looking for," Julian Radcliffe, chairman of the London-based Art Loss Register, which runs an international database of stolen and missing works, told AFP.
Investigators came upon the paintings during a 2011 search of an apartment belonging to the elderly son of art collector Hildebrand Gurlitt, who had acquired them during the 1930s and 1940s, according to Focus.
The search was carried out because the son, Cornelius Gurlitt, was caught by customs authorities on a train from Switzerland to Munich with a large amount of cash.
The collection uncovered included many of the masters of the 20th century, among them Pablo Picasso, Henri Matisse, Marc Chagall, Emil Nolde, Franz Marc, Max Beckmann and Max Liebermann.
A person who was present at the search told AFP that trash and discarded food packaging lay around the apartment and the paintings were stored on hand-built shelves hidden behind a curtain.
Gurlitt's father, despite having a Jewish grandmother, had become indispensable to officials in the Third Reich because of his art expertise and vast network of contacts.
Hitler's propaganda minister Joseph Goebbels put Gurlitt in charge of selling the art abroad.
However Gurlitt apparently secretly hoarded many of the works, and claimed after the war that the masterpieces were destroyed in the bombing of Dresden.
His son, a recluse without a job, had sold a few over the years, living off the proceeds, Focus said. For the moment he is only facing possible tax evasion charges.
The works are now stored in a customs warehouse outside Munich.
'Tip of the iceberg'
The head of the Central Council of Jews in Germany, Dieter Graumann, led calls for an exhaustive search for the provenance of the paintings, at least 200 of which were officially reported missing.
The spectacular find "makes clear once again that the Holocaust was not only a mass murder, it was a mass deadly hold-up," he told the WAZ newspaper group.
Anne Webber, founder and co-chair of the Commission for Looted Art in Europe, called the case "the tip of the iceberg".
"People have been looking for their looted art for 75 years now so if there are 1,500 paintings here it stands to reason that there are a lot of looted paintings that belong to families which should be returned to them," Webber told BBC television.
She said the fact that German authorities had still not published a list of the works or located a single rightful owner raised troubling questions.
"There's a culture of secrecy," she said.
Among the paintings discovered was a Matisse that had belonged to the Jewish collector Paul Rosenberg.
Rosenberg, who fled Paris leaving his collection behind, was the grandfather of Anne Sinclair, the former wife of the disgraced French politician Dominique Strauss-Kahn.
A Swiss gallery, Kornfeld, said it had made its last purchase from Cornelius in 1990, when it bought works on paper that were part of the Nazis' collection of "degenerate art" displayed in 1937 and bought by Gurlitt's father the following year.
"It is important to make a clear distinction between works that were looted and those seized by the National Socialists as 'degenerate art' which can be bought freely to this day," it said.
The Nazis plundered artworks in Germany and across Europe before and during World War II.
Thousands of stolen artworks have since been returned to their owners or their descendants, but many more have never resurfaced.
Webber said restitution efforts were of paramount importance.
"These were works that were taken from families whose lives were utterly destroyed or transformed by the Nazis, and so for them the return of this art is both justice and a form of reconnection to that life that was taken away from them," she said.