Art dealer Helly Nahmad is being sued for hiding a $20 million painting that had been stolen by the Nazis, according to the New York Post. It’s not been a great year for the Manhattan gallerist, who recently pleaded guilty to operating a gambling ring and was sentenced to a year and a day in jail (see artnet News report).

The Nahmad family claims not to own the painting, Amedeo Modigliani’s Seated Man with a Cane (1918), saying that it belongs to the International Art Center—which appears to be owned by Helly’s father, David Nahmad, and has roughly $3 to $4 billion in art holdings. Both men are now being sued by Philippe Maestracci, who says his grandfather, an art dealer named Oscar Stettiner, owned the piece, but was forced to leave it behind when he fled Paris during the Nazi invasion of 1939.

Amedeo Modigliani, Seated Man with a Cane (1918).

The suit is just the latest development in an ongoing dispute over the painting’s ownership. The Stettiner family had attempted to track down the work after the war, but it had been inaccurately labeled and the trail ran cold. In 2008, Maestracci learned that the lost artwork was being sold at Sotheby’s by the Nahmads. He then began his quest to have the painting, or its market value, returned. Worth roughly $20 million, the piece failed to find a buyer at auction.

In 2012, Maestracci’s lawyers argued that the “International Art Center is an offshore entity used by the Nahmad defendants as an instrumentality to hold their interests in works, around 90 percent of which are held in an art storage facility at the free port of Geneva,” (see report from the Art Newspaper).

In the current suit, Maestracci hopes to unmask the “murky corporation” (as per Hyperallergic), determining who the painting actually belongs to, so that he can get it back.

UPDATE: artnet News spoke with Helly Nahmad’s lawyer, Richard Golub, who pointed out that the details of the 2011–12 case, which are a matter of public record, provide a detailed account of the painting’s whereabouts since 1996. This latest action is only a summons, and a new complaint has not been filed. artnet News has since published a follow-up story.