They were less clear on why he kept his windows covered, or why every so often a man in an expensive car would come to the house carrying paintings to, not from, a painter.

“He would bring a painting in and show it to him, for him to work on or fix up something,” Edwin Gardiner, 68, who lives across the street, said before pausing and adding, “I don’t know what he did with it.”

Parts of the mystery became clearer on Friday as neighbors learned that Mr. Qian, a quiet 73-year-old immigrant from China in a paint-flecked smock, is suspected of having fooled the art world by creating dozens of works that were modeled after America’s Modernist masters and were later sold as their handiwork for more than $80 million.

Mr. Qian, who came here more than four decades ago and struggled to sell his own works in this country, earned just a few thousand dollars for each of his imitations. New York was a center of the art world, but Mr. Qian told friends that he had been disheartened by the difficulties he encountered finding a foothold as an artist.

“He was kind of frustrated because of the language problem, the connection problem,” said Zhang Hongtu, a friend and acclaimed Chinese artist who lives in New York. “He was not that happy.”

Mr. Qian’s low profile as a painter has evaporated; federal authorities have decided that he was the artist whose fakes are at the heart of one of the bolder art frauds in recent memory.

A revised federal indictment handed up this week charged Glafira Rosales, one of the dealers accused of having peddled his works, with money laundering and tax evasion in connection with the sales.

The indictment does not name Mr. Qian, who could not be reached for comment on Friday, but people with knowledge of the case confirmed that he was the man referred to only elliptically in the charges as the “Painter.” Neighbors say agents with the Federal Bureau of Investigation searched his home this week.

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.

So far, Ms. Rosales is the only person who has been charged in connection with what prosecutors have described as a long-running fraud. They have not indicated that either Manhattan dealer was aware that the works, which have been displayed at international exhibitions, a museum and an American embassy, had recently been created across the East River.