Previously unknown paintings by Henri Matisse and Otto Dix are among a vast trove of Nazi-looted art found in a Munich apartment that includes works by some of Europe's most celebrated artists, German experts said on Tuesday.
Customs investigators seized the 1,400 art works, dating from the 16th century to the modern period and by artists such as Canaletto, Courbet, Picasso and Toulouse-Lautrec, last year, an official said.
They had remained silent until now not because of any "improper intentions", they added, but because they had chanced upon the art during a tax evasion probe, which compels secrecy.
While experts consider the works to be of huge artistic value, the task of returning them to their rightful owners could take many years and poses a huge legal and moral problem for German authorities.
The haul, found in the flat of Cornelius Gurlitt, the reclusive son of a war-time art dealer, is one of the most significant discoveries of works seized by the Nazi regime. It could be worth more than 1 billion euros ($1.3 billion), according to a German magazine, although officials declined to comment.
Gurlitt, who occasionally sold paintings to support himself, has since vanished.
The paintings, which were found in generally good condition, are being stored in an undisclosed location and no list will be published - something that has been criticised by those seeking to recover lost art. The decision may be intended to deter false claims that would distract expert investigations.
"When you stand in front of works that were long considered lost, missing or destroyed, and you see them again, in a relatively good condition - a little bit dirty but not damaged - it's an incredible feeling of happiness," said Meike Hoffmann, an art expert from Berlin's Free University who has been assessing the find.
Hoffmann said that among the previously unknown paintings was a self-portrait by Dix, in impeccable condition, and probably painted around 1919.
A similarly unknown Matisse painting, of a seated female figure that he had painted several times, probably dated from the mid 1920s and was confiscated in 1942. There was also a work by Marc Chagall not previously known.
Slides of the works were shown during a news conference, including the Matisse and a group of horses by German expressionist Franz Marc.
The Nazis systematically plundered hundreds of thousands of art works from museums and individuals across Europe. Thousands of works are still missing.
Investigators made the spectacular find after Gurlitt, believed to be in his seventies, aroused their suspicions as he travelled by train between Zurich and Munich, with a large sum of cash, according to German media.
Jewish groups have urged that the origins of the art works be researched as quickly as possible, so that, if looted or extorted, they can be returned to their original owners.
For some families missing art constitutes the last personal effects of relatives murdered during the Holocaust.
"Had this discovery been made public at the time it was made, families looking for their lost art would have been able to potentially identify works within this collection," said Julius Berman, Chairman of the Conference on Jewish Material Claims Against Germany.
"Publicizing the existence of Nazi-looted art is essential to the process of finding heirs," he added.
The group cited an agreement struck in Washington in 1998, where 44 governments endorsed a set of principles for dealing with Nazi-looted art, including that every effort should be made to publicise it.
Besides paintings the haul included a large number of drawings and pastels on paper.
"We were able to confiscate 121 framed art works and 1,285 non-framed works, including some famous masterpieces," Nemetz said. "We had concrete clues that we were dealing with so-called 'degenerate art', or so-called looted art."
Cornelius's father Hildebrand Gurlitt was, from 1920, a specialist collector of the modern art of the early 20th century that the Nazis branded as un-German or "degenerate" and removed from show in state museums, or displayed simply to be mocked.
Nazi propaganda minister Joseph Goebbels recruited Gurlitt to sell the "degenerate art" abroad to try to earn cash for the state. Gurlitt bought some for himself and also independently bought art from desperate Jewish dealers forced to sell.
Investigators said the collection comprises works which are clearly from the Nazi regime's state-owned collection of "degenerate art". Others, which may have had several owners or may have been extorted from owners fearing Nazi persecution, will need extensive research.
Jonathan Petropoulos, a history professor at Claremont McKenna College in Claremont, California, and author of "The Faustian Bargain: The Art World in Nazi Germany", said: "Hildebrand Gurlitt became a dealer for Hitler and went to the Nazi art-looting headquarters in Paris where he presumably got a lot of works."
Gurlitt, who fled to the West after the war, claimed he had lost all his art and papers in the bombing of Dresden. "Obviously that was a lie," Petropoulos added.
Germany has faced criticism that the restitution process is too complicated and lacks sufficient funding.
Restitution groups and lawyers have often criticised state and museum authorities for not doing enough to research works' origins themselves and instead leaving the onus on relatives.