Even before an immensely wealthy and anonymous bidder secured Van Gogh's portrait of Dr Paul Gachet for £48.8 million at a Christie's auction in 1990, its ownership was in dispute.
It was hanging in the Stadtische Museum in Frankfurt in 1936 when the Nazi, Hermann Goering, described it as "degenerate" before he removed it from the wall, but curiously kept it in his bedroom for a year.
He sold it a year later through a German art dealer to the Amsterdam banker, Franz Koenigs, who sent it to his friend Dr Siegfried Kramarsky in New York for safe keeping. By 1949 Dr Kramarsky was exhibiting the painting as his own after Koenigs was murdered.
Now, in a new twist to the tale, Koenigs descendants are preparing a legal suit to have the Van Gogh returned to them.
The Koenigs' trawl through their history is mirrored, albeit on a larger scale, by the National Gallery's decision to take a close look at its records to establish whether any of its paintings could have been looted or stolen during the Nazi regime, in particular the years between 1933 and 1945. For much of the past three months the National Gallery has tracked the provenance of 120 paintings whose whereabouts at particular periods between 1933 and 1945 are unknown or uncertain.
Researchers and historians have been asking private collectors, the executors of wills and descendants of previous owners to trace the histories of such paintings as Monet's La Pointe de la Heve Sainte-Adresse, Delacroix's Christ on the Cross and Courbet's Still Life with Apples. The gallery has only 2,400 paintings in its collection compared to the British Museum.
It has seven million objects, although not all are paintings. It is a mighty task.
While 470 of its paintings were acquired after 1933, the National Gallery has yet to uncover direct evidence that the Nazis stole any of the 120 "suspect" paintings.
It does have very strong suspicions about 10 of them, including a Caravaggio, a Van Dyck and a Picasso, all of which disappeared completely from art records or were unknown at the height of the Nazi occupation of Europe, but some as early as 1933.
Stephen Ward, the associate director of the Holocaust Educational Trust in London, a research and education body, says that in that year many Jews in Germany were forced to sell their art treasures, and the practice was then implemented in Austria and in eastern Europe as the German army advanced.
To further complicate matters, not only had the paintings been sold 30 or 40 times since then, but when the Russian army overran Berlin they removed looted art treasures and took them back to Moscow, and the trail went cold.
The fact that some of these treasures may have ended up in British galleries by way of disreputable art dealers in Europe was a matter of some concern to the chairman of the Holocaust Educational Trust, Lord (Greville) Janner, a former president of the Board of Deputies of British Jews, who is now a working Labour peer.
He proposed an audit of museum collections to the Culture Secretary, Mr Chris Smith, last spring, and the government agreed. But so far the National Gallery is the only national museum to take a close look at its books. Municipal and private galleries have yet to indicate publicly their willingness to take part.
The Holocaust Educational Trust also wrote to the Ministry of Defence and Mr Smith's department to ensure the Government Art Collection of 12,500 paintings in 600 different buildings in 300 different countries did not contain any looted art.
The government art audit is complete and the collection has been given a clean bill of health, since the majority of paintings are 20th-century pieces.
But there are gaps in the MOD's records. However, it is almost certain that its collection of portraits of Queen Elizabeth hanging in far-flung outposts of the Commonwealth will not be exposed as Nazi loot, even if the Allied forces in Europe often treated "Nazi art" as legitimate reparation, rather than restoring paintings to their rightful owners. In his research of Britain's post-war role in handling looted art and settling restitution claims, Mr Ward says that just as the Koenigs have discovered, the search for a looted painting is a "uniquely emotional" experience.
Paintings sold under pressure to pay for the family's flight from occupying German forces represent their only link with home.
What is certain is that some European governments are much more willing to address complex issues, such as looted art and compensation for slave labour ers, than they were 10 or 20 years ago. In some cases they have been prompted by the threat of multi-million-pound lawsuits: in others there is a genuine wish to make amends.
For the real owners of looted art it is a simple question of truth: "Some people want money in compensation for stolen art; others want a plaque on the wall beneath the painting to say that the Nazis stole the painting. Some just want to be able to go and look at it."