These problems have permeated the industry for decades, said Stephen Urice, a professor at the University of Miami School of Law. He pointed to a 1978 court ruling: “In the fantasy land of marketing in the fine arts,” prestigious names are “dropped freely as rain,” and large sums quickly change hands, Judge J. Shorter of New York State Supreme Court wrote. “In an industry whose transactions cry out for verification of both title to and authenticity of subject matter, it is deemed poor practice to probe into either.”

Image Ann Freedman, gallery chief. Credit... Fred R. Conrad/The New York Times

In the Knoedler case, the only person convicted so far is Glafira Rosales, a Long Island dealer who pleaded guilty to fraud last fall. Prosecutors say her co-conspirators included her boyfriend, Jose Carlos Bergantiños Diaz; his brother, Jesus Angel Bergantiños Diaz; and Pei-Shen Qian, the artist who forged the works. The Bergantiñoses have been released on bail in Spain, while Mr. Qian is in China. The dealers who sold dozens of these works — Ann Freedman, the former president of Knoedler, and Julian Weissman — have repeatedly said that despite the lack of documentation, they were convinced the art was genuine.

Jack Flam, an art historian who was one of the first to challenge the authenticity of the Rosales paintings, said: “If you asked me what the biggest factors were behind this thing succeeding so long, first is that everybody was afraid to be sued. People give credibility to works unwittingly by keeping quiet.”

Mr. Flam is the president of the Dedalus Foundation, a nonprofit group created by the painter Robert Motherwell. He and his colleagues were initially impressed by the supposed Motherwells, but by late 2007, as the number of new discoveries attributed to Motherwell grew, they became uneasy. Mr. Flam began making inquiries and soon discovered that the Richard Diebenkorn estate, the Willem de Kooning Foundation and the Barnett Newman Foundation were also suspicious of works that Knoedler was selling.

“The foundations all knew that something was wrong with their particular artist but didn’t know anybody else had problems,” Mr. Flam said. “I was the first one who went around and starting asking everyone.”