A museum in Switzerland is set to remove five famous paintings from one of its exhibitions while it investigates whether they were looted by the Nazis.

The Kunsthaus Zurich Museum said the decision to remove the paintings comes after the publication of new guidelines aimed at dealing with the art pieces that have still not been returned to the families they were stolen from during World War II.

The pieces are part of the Emil Bührle Collection, which was named after a German-born arms dealer who made his fortune during World War II by making and selling weapons to the Nazis.

The pieces under investigation are “Jardin de Monet à Giverny” by Claude Monet, “Portrait of the Sculptor Louis-Joseph” by Gustave Courbet, “Georges-Henri Manuel” by Henri de Toulouse-Lautrec, “The Old Tower” by Vincent van Gogh, and “La route montante” by Paul Gauguin.

The foundation board for the Emil Bührle Collection said in a statement it was “committed to seeking a fair and equitable solution for these works with the legal successors of the former owners, following best practices.”

Earlier this year, 20 countries including Switzerland agreed to new best practices from the U.S. State Department about how to deal with Nazi-looted art. The guidelines were issued to mark the 25th anniversary of the 1998 Washington Conference Principles, which focused on making restitution for items that were either stolen or forcibly sold.

Stuart Eizenstat, the U.S. Secretary of State’s special advisor on Holocaust issues, said in March that as many as 600,000 artworks and millions of books and religious objects were stolen during World War II “with the same efficiency, brutality and scale as the Holocaust itself.”

“The Holocaust was not only the greatest genocide in world history,” he said during an address at the Holocaust Museum in Washington D.C. “It was also the greatest theft of property in history.”

According to the CBS News partner BBC, the principles are an important resource for families seeking to recover looted art because, under Swiss law, no legal claims for restitution or compensation can be made today for works from the Bührle collection due to the statute of limitations.

A sixth work in the collection, “La Sultane” by Edouard Manet, also came under further scrutiny, but the foundation board said it did not believe the new guidelines applied to it and that the painting would be considered separately, the BBC reported.

“Due to the overall historical circumstances relating to the sale, the Foundation is prepared to offer a financial contribution to the estate of Max Silberberg in respect to the tragic destiny of the former owner,” the foundation said.

Silberberg was a German Jewish industrialist whose art collection was sold at forced auctions by the Nazis. It is believed he was murdered at Auschwitz, a Nazi death camp during the Holocaust.