A city in western Germany said Wednesday it is opening an investigation into eight disputed works by Dutch artist Piet Mondrian that have been part of one of its museums' collections for decades.

A spokesman for the city of Krefeld, Christoph Elles, said the Kaiser Wilhelm Museum strongly rejects allegations by lawyers for Mondrian's heirs that the works' presence at the museum had been deliberately concealed. He said the museum believes the artist himself donated eight works in 1929, though there is no clear proof for that.

A lawyer for the heirs, Gunnar Schnabel, questioned whether the artist ever donated the works to the museum and said it illegally sold four of them. "The fact that Krefeld had eight works from (Mondrian's) most important creative period ... was concealed until the summer of 2016," Schnabel wrote in a statement emailed to The Associated Press on Wednesday.

The New York Times first reported about the dispute on Sunday. Mondrian, one of the most important artists of the 20th century, is famous for his abstract compositions of black lines and rectangles filled with bright colors.

The museum said eight of his works surfaced there in 1950. While it is not clear how they came there, the museum said it's possible that the artist himself gave them the works in 1929, for an exhibition that never took place. Mondrian never reclaimed the works and after 1950, the museum swapped four of the artworks for works by other artists, it said. The remaining four Mondrian works are still in the Krefeld museum's possession. Schnabel, the lawyer for the Mondrian heirs, said it would have been "hopeless" for Mondrian to try to reclaim his works because they were deemed "degenerate art" after the Nazis took power in Germany in 1933. Mondrian himself left Europe for the United States and died in exile in New York in 1944. The museum said it would task an expert with shedding light on the disputed history of the eight works.