Two Jewish heirs to Nazi-era art dealers are suing Germany and a German museum to return a medieval art collection with a estimated worth of $226 million. The lawsuit filed Monday in the U.S. District Court in Washington, D.C., says the Welfenschatz, also known as the Guelph Treasure, was stolen in a “sham transaction” from their predecessors in 1935.

"The Jewish people who owned this art had their property squeezed out of them while their lives and the lives of their families were at risk,” said Nicholas M. O’Donnell of Sullivan & Worcester LLP, an attorney for the plaintiffs, Alan Philipp from London, England, and Gerald Stiebel from Santa Fe, New Mexico. "The value of that collection was four to six times as much as these victims were paid.”

The Guelph Treasure includes dozens of reliquary art pieces dating back to the 11th to 15th centuries. The collection was first owned by the Prussian aristocrats from the House of Brunswick-Luneberg. In 1929, the collection was sold to Julius and Selig Goldschmidt, Isaak Rosenbaum, and Z.M. Hackenbroch -- Jewish art dealers who are ancestors to the plaintiffs in the case. In the 1930s, the lawsuit claims the collection was sold in a coerced transaction to one of Adolf Hitler’s top deputies, Hermann Göring. The lawsuit claims the works were sold at a fraction of what the collection was worth and the price was further marked down after the Nazis charged the men “flight taxes” to help them flee Germany.

The German museum, the Prussian Cultural Heritage Foundation, argues the sale was not pressured and the collection was sold at a fair market price. In 2014, a German government advisory commission agreed with that interpretation. In a ruling it concluded the collection was not a "forced sale due to persecution" and should remain in Germany.

The latest lawsuit invoked the U.S. Foreign Sovereign Immunities Act, a U.S. law that allows a lawsuit to be brought against foreign states that conduct business with the United States. Hermann Parzinger, the head of the Prussian Cultural Heritage Foundation, does not believe the U.S. lawsuit will yield a different result than what took place in Germany.

"While we believe that there is no jurisdiction over this claim in the United States, we are confident that any court ruling on the merits would reach the same conclusion that we and the advisory commission have reached," Parzinger told the Associated Press on Tuesday.

The collection, which has been on display at Berlin’s Museum of Decorative Arts, has been named a national cultural treasure. By German law it cannot leave the country without permission from Germany’s cultural minister, Monika Grütters. However, Grütters has not necessarily shied away from restitution cases. Last month she created the German Center for Lost Cultural Property with a $6.8 million budget to help smaller museums find the provenance of artwork during and after Hitler’s rule. Some museums have been slow to cooperate, fearing that digitizing their records could compromise their privacy, the Wall Street Journal reports.

Gerald Stiebel, one of the plaintiffs in the latest lawsuit, is the descendent of Isaak Rosenbaum -- one of the original owners of the Guelph Treasure.

"My great-uncle Isaak was fortunate to make it to Amsterdam, but my father always talked about how the family had to leave so much behind." Stiebel said. "We’ve been looking for justice for years from today's German government. They claim the Nazis bought it in a fair marketplace. How is that possible? We hope we can find justice now in the United States court system."