Piet Mondrian’s heirs have laid a claim to four of the artist’s paintings currently held by the Kaiser Wilhelm Museum in Krefeld, Germany.

Heirs of the famous modernist painter claim they are the rightful owners of the works, which Mondrian left behind when he fled Europe in the 1930s. Over three years, a team of experts hired by the heirs—led by provenance researcher Monika Tatzkow—investigated the pieces and found that the works were first exhibited in 1929. They were then loaned to the museum in Krefeld, along with four additional paintings by the artist, an account seemingly backed up by a 2010 email from a now-retired curator at the museum. But the disputed works didn’t show up in an official museum inventory until 1954, under what the then-director called “mysterious circumstances.” Although the Wilhelm has since claimed that Mondrian gifted them the works, “it has been unable to buttress that claim with evidence,” the New York Times reported. Tatzkow told the Times it seems unlikely that Mondrian would gift works at such a perilous moment, and that if they were donated, they should have been inventoried. “The theory that these were a gift is completely absurd,” she said. The museum has also argued that any legal claim by the heirs is time-barred by the statute of limitations governing the dispute.