The catch, however, is that the Nazis may have obtained the paintings legally.

Unlike in many other restitution cases, an unknown number of the works the Katz heirs are seeking to recover were not confiscated but sold, possibly at market value.

Katz’s daughter, Sybilla Goldstein-Katz, who lives in Florida, said through a lawyer that the works were sold to obtain safe passage from the Nazis. For instance, a Rembrandt secured the release of Katz's mother from the Westerbork concentration camp, one newspaper reported.

"He had to do business with them; the pistol was to his nose," one of Katz's other children, 88-year-old David Katz of Switzerland, said of his father's dealings with the Nazis.

However, a prominent Dutch looted-art expert, Rudi Ekkart, who helped create the standards used by a committee that advises the Dutch government on art restitution, said some of the paintings listed in the claim were sold before World War II even began.

The trove is particularly valuable, curators say, with prized paintings by the 17th century artists Jan Steen, Gerard Dou and Jacob van Ruisdael.

One leading Dutch newspaper, NRC Handelsblad, reported that Katz sold the paintings voluntarily and at fair price to Alois Miedl, a Katz friend from Germany who became the buyer for Nazi leader Hermann Goering.

The Katz claim is "absolute nonsense," said Gerart Aalders, a historian at the Institute for War Documentation in Amsterdam who has written three books on the Nazi pillaging of Jewish property in Holland. "The archives clearly show that Katz willingly sold works to a friend under no duress. He also continued to sell to the Germans even after he had escaped to Switzerland."