According to the indictment and other court papers, Mr. Qian was discovered selling his art on the streets of Lower Manhattan in the early 1990s by Ms. Rosales’s boyfriend and business partner, an art dealer named Jose Carlos Bergantiños Diaz, who recruited him to make paintings in the style of celebrated Abstract Expressionists. The indictment does not name Mr. Bergantiños Diaz, but his identity is confirmed by other court records.

Over a period of 15 years, court papers claim, the painter, working out of his home studio and garage, churned out at least 63 drawings and paintings that carried the signatures of artistic giants like Jackson Pollock, Barnett Newman, Robert Motherwell and Richard Diebenkorn, and that Mr. Bergantiños Diaz and Ms. Rosales boasted were authentic. They were not copies of paintings, but were sold as newly “discovered” works by those artists.

Ms. Rosales sold or consigned the art to two elite Manhattan dealers, Knoedler & Company — New York City’s oldest gallery until it closed in 2011 — and a former Knoedler employee named Julian Weissman, who in turn sold them for millions of dollars to customers who placed their faith in the dealers’ reputations and the supporting words of some experts.

Ms. Rosales told the dealers that the vast majority of the paintings came from a collector who had inherited the works from his father and adamantly refused to be identified. Over time, this anonymous owner came to be referred to as “Secret Santa” and “Mr. X.”

Knoedler, its former president Ann Freedman and Mr. Weissman have repeatedly said they always believed the works to be authentic, despite the lack of documentation.

[...] the saga began in the early nineties when Rosales approached Freedman with a fabulous tale. She claimed to represent a foreign collector who “was of Eastern European descent, maintained residences in Switzerland and Mexico, wished to remain anonymous, and had inherited the works ... from a relative.” Based on this account, Freedman came to call the relative “Mr. X” and the anonymous seller “Mr. X, Jr” — of course, neither existed.

“The story was credible,” said Freedman, who is tall with tightly curled silver hair and a controlled, energetic manner. “Dealers often do not know the specifics of origin or background, or how the art left the artist’s studio. You cannot turn the pages of an auction catalogue or museum publication without seeing a majority of the works labeled ‘private collection.’ The chain of ownership is often out of order and incomplete.”

With time, the story of how Rosales obtained the paintings became more complex: The married Mr. X and David Herbert, a real-life gallery employee and owner who died in 1995, knew each other in the fifties; they were lovers, and Herbert offered Mr. X access to the studios of the Abstract Expressionists, through whom he could purchase paintings off the books; these works were hidden away until Herbert’s death to protect Mr. X’s secret. (Herbert, of course, had nothing do with any Mr. X or secret stash of paintings — since neither existed.)

Freedman says that she did her best to get answers from Rosales. “I went to Glafira and pushed and pushed to get more information, relentlessly,” Freedman said. "My ongoing diligence met more than the gold standard; there is plenty of evidence of that.”