WASHINGTON (AP) — The Supreme Court on Thursday kept alive a California man’s hope of reclaiming a valuable impressionist masterpiece taken from his family by the Nazis and now on display in a Spanish museum.

The question in the case was not directly about whether San Diego resident David Cassirer can get back the streetscape by French impressionist Camille Pissarro. Instead, the question was how to determine whose property laws — Spain’s or California’s — ultimately apply to resolving the dispute over “Rue Saint-Honoré in the Afternoon, Effect of Rain.” The painting of a Paris street with horse-drawn carriages and a fountain is now worth tens of millions of dollars.

Lower courts had concluded Spanish property law should ultimately govern the case and that under Spanish law, the museum was the rightful owner of the painting the family believed for over half a century had been lost or destroyed. If the Supreme Court had upheld that ruling, the case would have been over. Instead, the justices unanimously sided with Cassirer and sent the case back to lower courts, where he still faces hurdles to getting the painting back.

The museum and Cassirer could also come to some agreement rather than continue to fight in court.

Justice Elena Kagan wrote in the court’s 9-page opinion that the “path of our decision has been as short as the hunt for Rue Saint-Honoré was long; our ruling is as simple as the conflict over its rightful owner has been vexed.”

In statements, Cassirer’s lawyers cheered the decision. David Boies, who argued the case before the justices, called it “a great day for the Cassirer family and for all who care about justice.”

Cassirer’s lawyer Scott Gant said he hoped that Spain and the foundation that runs the museum for the Spanish government would “reflect, and conclude they should return the painting rather than maintain their longstanding refusal to do what is just.”

In ruling in the case, lower courts have criticized Spain for not living up to commitments to return Nazi-looted art.

Lawyers for the museum did not immediately respond to messages seeking their reaction to the decision.

The story of the stolen Pissarro painting goes back to Cassirer’s great-grandmother Lilly Cassirer, a German Jew. She had owned the 1897 oil painting, one of a series of 15 that Pissarro painted of a Paris street as seen from his hotel window.

After the Nazis came to power and years of intensifying persecution, Lilly Cassirer and her husband decided to flee Germany. In 1939, in order to get visas to leave, she surrendered the Pissarro painting to the Nazis.

The painting changed hands a number of times after that. It is now in the collection of the Thyssen-Bornemisza National Museum in Madrid, which has fought to retain it. It has been said to be worth more than $30 million.

In 1958, Lilly Cassirer reached a monetary settlement with the German government worth about $250,000 in today’s dollars, but she didn’t give up rights to try to pursue the painting if it turned up.

In fact, the painting was not lost or destroyed but had traveled to the United States, where it spent 25 years in the hands of different collectors before being purchased in 1976 by Baron Hans Heinrich Thyssen-Bornemisza of Lugano, Switzerland. He owned it until the 1990s when he sold much of his art collection to Spain for more than $300 million. The Thyssen-Bornemisza National Museum, a renovated palace, now houses the collection.

In 2000, Lilly Cassirer’s grandson Claude Cassirer learned the painting’s whereabouts. Spain rejected his attempts to get it back, however, and he ultimately sued in his home state of California in 2005. Claude Cassirer died in 2010. It’s his son David now fighting for the piece’s return.

In her opinion, Kagan wrote that in lawsuits like Cassirer’s, foreign parties should be treated just as a private party would be. The “standard rule” there is that the rules of the state where the lawsuit is filed should apply, she said.

That means lower courts should have looked to California rules about what law should be used to resolve the case. That doesn’t necessarily mean Cassirer will win. One lower court said that under California rules — which require weighing whether California or Spanish property law should apply — Spanish law would still apply and the museum would win.

Kagan, one of two Jewish justices on the court, ended the opinion by attaching an image of the painting and an image of the painting hanging in Lilly Cassirer’s living room in Germany.

The case is David Cassirer v. Thyssen-Bornemisza Collection Foundation, 20-1566.

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