Art dealer Guy Wildenstein should get a four-year prison sentence and pay a 250 million-euro ($276 million) fine for concealing paintings in an attempt to avoid taxes, French prosecutors said in their closing speech at a month-long fiscal fraud trial.

Guy Wildenstein fraudulently hid assets worth hundreds of millions of euros in offshore trusts to underestimate inheritance taxes, prosecutor Monica d’Onofrio said Thursday at the Paris criminal court. Two years of the prison sentence should be suspended, she said.

“This is the most sophisticated tax fraud” seen in France for many decades, d’Onofrio said. “It’s thanks to the Wildenstein fiscal fraud we’ve learned the impressionist art of defrauding the tax collector.”

The Paris trial, which will last until Oct. 20, has provided insight on the family’s business secrets, which were so fiercely held that Guy Wildenstein said he didn’t learn of many of the financial machinations until the death of his father Daniel in 2001.

According to a banker who has intricate financial knowledge of the clan’s dealings, the Wildensteins sold more than 600 pieces since the turn of the century, generating around $300 million in cash to fund their lifestyle. The family has works worth nearly three times that much in storage.

Daniel Wildenstein created an offshore trust in the Bahamas in 1998 to lodge about 2,500 works from his collection. A Royal Bank of Canada unit managed it, and a representative told a Paris court last month that its sole purpose was to provide funds for the family.

The assets held in trusts weren’t legally Daniel’s, Guy’s lawyers have said. Instead, they belonged to the trusts and therefore shouldn’t count for estate taxes. French prosecutors argue that the trusts aren’t truly independent, pointing to evidence that the Delta Trust became a source of bounty for Guy and his brother Alec, who died in 2008.

“The purpose of the trusts has been perverted and they became piggy banks,” d’Onofrio said. Her colleague, Mirelle Venet, argued that Guy’s nephew, Alec Junior Wildenstein, had a marginal role in the tax fraud and should therefore get a six-month suspended sentence.

The Wildenstein family entered the art world in the 1870s in Paris when Nathan Wildenstein, Guy’s great-grandfather, helped a client sell some paintings while he was working as a tailor. Nathan opened his own gallery the same decade. His firm, Wildenstein & Co., has been family-run since then.

Five generations later, the art-dealing tradition continues through Guy, in charge of the Wildenstein Institute, whose ‘catalogues raisonnes’ for the most important artists of the 19th and 20th centuries are so exhaustive that a work by Monet would be worthless without a so-called Wildenstein index number.

This article was provided by Bloomberg News.