In September 2010, customs officials stop an elderly man in a train from Zurich to Munich. His name: Cornelius Gurlitt. He is carrying a suspiciously large amount of money with him. Even though that's not forbidden, border guards report him to tax officials. In 2011, a search in his Munich apartment leads to an astonishing discovery.

Among junk and expired cans of food littering the residence, the investigators discover thousands of art works which had gone missing during World War II. Behind this invaluable art collection are countless episodes of deep pain and injustice.

The art works are presumed to have been looted by Nazis, which confiscated so-called "degenerate art" and stole works from Jewish collectors. Paintings by Pablo Picasso, Henri Matisse, Marc Chagall, Paul Klee, Max Liebermann (pictured: "Two Riders on the Beach"), as well as other modern classic paintings, make up the collection. The name Gurlitt was already notorious in Germany's art world.

Hildebrand Gurlitt, Cornelius Gurlitt's father, was an art dealer during the Nazi era. He traded "degenerate art" for the planned Führer Museum in Linz and sold paintings which had been confiscated from Jewish owners or museums abroad. After the war, he reported that his private collection and all business records had disappeared in the Dresden fire bombing of 1945.

In 2011, his son Cornelius Gurlitt sells Max Beckmann's painting, "The Lion Tamer." A 1931 label of the Flechtheim Gallery glued on the back of the painting should have made the art world suspicious. This Jewish art dealer had fled Germany in 1933, leaving many art works behind. The painting was auctioned for 864,000 euros (about $1 million).

In November 2013, the seized art collection in Munich is made public. Since 2011, the art historian Meike Hoffmann (pictured) has been in charge of determining where the paintings come from and finding their owners. It's an extremely complicated task, charged with moral, legal and historical questions.

Provenance researchers like Meike Hoffmann search for the origin and history of a painting. Labels on the back of the art works or business records from art dealers and auction houses can provide clues. Cornelius Gurlitt inherited the paintings from his father, yet descendants from Jewish families claim to be the legitimate owners and want them back.

According to German law, the Gurlitt case is clear: The owners' right to reclaim Nazi-looted art expired after 30 years. The Bavarian Ministry of Justice proposed to change the law surrounding this time limitation in cases where people knowingly possessed looted art. The draft is still being examined.

The German and Bavarian governments have promised to clear up all aspects of the Gurlitt case, setting up a special task force. Until now, little has happened - only two paintings have been identified as Nazi-looted art. The experts led by Ingeborg Berggreen-Merkel are still searching for the origins of the paintings. Some of the alleged heirs are elderly and may not live to see the result.

David Toren, 89, is suing Germany and Bavaria. He wants the painting "Two Riders on the Beach" back. It had once belonged to his great-uncle David Friedmann, who was killed by the Nazis in 1942. Descendants of Jewish collectors whose art was confiscated are growing impatient.

New headlines shake the art world. Cornelius Gurlitt's lawyers uncover a series of paintings by Monet, Manet, Corot, Courbet and Renoir in a seemingly abandoned house belonging to the Gurlitt family in Austria. The art works are claimed to be part a private collection. Could these also be Nazi-looted art?

Months later, Cornelius Gurlitt strikes a deal with the German government. He is ready to allow all paintings suspected of being Nazi-looted art to be subjected to provenance research. In exchange, he gets back the works seized in 2012. On May 6, Cornelius Gurlitt dies at the age of 81 in Munich. But the thriller goes on.

Surprisingly, Cornelius Gurlitt bequeaths his collection to the Museum of Fine Arts Bern. He mentions in his will good memories of time spent in Switzerland. The German government greets this last will, but a part of the Gurlitt family feels bypassed.

Two of Gurlitt's cousins, who were not mentioned in his will, commissioned a psychiatric report to determine whether he was in his right mind when writing the legal document. The report concludes that he was not; however, the will is not contested. On November 24, the Museum of Fine Arts Bern confirms that it would accept Gurlitt's collection.