Piet Mondrian’s heirs called on a German city to restitute four of his paintings.

Piet Mondrian’s heirs and the German city of Krefeld are locked in a dispute over four paintings by the pioneer of Neo-Plasticism and De Stijl that have been held by a city-owned museum for about 90 years. The heirs assert that the paintings were loaned to the city for an exhibition that had been planned for the summer of 1929, but was ultimately cancelled. The city has claimed it believed the works had been a gift from Mondrian, and furthering that claim is the fact that the artist—who fled Europe during World War II and died in New York in 1944—does not appear to have requested their return. Yet according to a New York Times report, neither party has irrevocable proof to support their claim.

The four paintings were made in 1925 and 1926, and belong to Mondrian’s most famous body of work, the geometric compositions of lines and rectangles painted in primary colors, black, white, and gray. According to the heirs’ report, Mondrian originally loaned at least eight paintings to the city museum, but its director sold or traded half of them in the 1950s. The four paintings that remain weren’t logged in the inventory of the museum’s collection until 1954.

Mondrian’s heirs have not filed a formal claim for the works’ return, but according to Krefeld, such a claim would be time-barred because, under German law, the statute of limitations for such action has passed. The heirs riposted that their claim could be filed in the United States, where it would not be barred.