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.

Long before Mr. Qian settled in Queens, he worked as an artist. He grew up in the island city of Zhoushan and in Shanghai, according to a 2004 interview with a Chinese television station. When he was in his 30s, Mr. Qian took part in a daring experimental art movement in Shanghai in the late 1970s as the Cultural Revolution was ending.       

Abstract art was considered a symbol of bourgeois decadence, but Mr. Qian and his fellow artists organized their own unofficial exhibition with work that employed abstracted forms. A 1987 exhibition at the Community Folk Art Gallery in Syracuse revisited that period with a show of six artists that included Mr. Qian.

In 1981, Mr. Qian came to New York, where he and his friend Mr. Zhang took art classes together at the Art Students League on West 57th Street. Although they were out of touch for about 20 years, Mr. Zhang recounted how Mr. Qian, a few years older, helped him find his way in an alien, overwhelming city.

“He told me how to make a living over here,” Mr. Zhang said, meaning that immigrants in their circumstances would have to take other jobs to support themselves.

“We worked together for short time at a construction site,” he said. He remembers Mr. Qian trying to sell his art on the street.

By the 1990s, Mr. Qian was becoming increasingly disenchanted with his own work. A fellow artist, Chen Danqing, recalled that period in an article he wrote for a Chinese art magazine in 2006. “The things we had done before became like dust in the wind,” he wrote. He described his friend, then in his 40s, as “homesick” and “lost.”

Mr. Qian is represented by a gallery in China. But in recent years, his neighbors said, he still spoke frequently, unprompted, of his dissatisfaction with his career in the United States.

“When he was over in China he would feel like a rock star, because when he would walk out on the street everybody knew him,” Mary Ann Gardiner, Edwin Gardiner’s wife, recalls Mr. Qian saying. “It was like night and day.”

Mr. Zhang was shocked to hear the allegations of Mr. Qian’s involvement in what appeared to be a far-flung art fraud, saying the Mr. Qian he knew was always honest and hard-working. “I can’t believe it,” he said.

It is unclear how much Mr. Qian knew of the fraud that federal investigators have described. It is not against the law to make a replica of a masterpiece or sign the name of someone like Rembrandt and sell it, as long as it is clearly identified as a fake. What is illegal is to market it as authentic.

It is also unclear exactly who was delivering the paintings to Mr. Qian that the neighbors saw, or whether they were originals or perhaps just posters of works by famous artists.

Mr. Zhang was skeptical that his friend could have mimicked the style of such different and well-known artists. His memory of Mr. Qian’s work was of landscapes and interiors that used the pastel palette of the Impressionists.

“I didn’t know he had this kind of a good technique,” he said. “He had some talent, but I don’t believe he can paint in the same style as a Jackson Pollock; it’s not easy to copy this kind of style.”

Richard Grant, the executive director of the Diebenkorn Foundation, said, “Whoever did this was extremely skillful.”

It is not impossible, however. The German forger Wolfgang Beltracchi, convicted in 2011, says he created hundreds of paintings ostensibly by 50 artists.

According to the timeline laid out in the indictment, Ms. Rosales began showing the painter’s fakes to Knoedler and Mr. Weissman around 1994.

The scheme began collapsing in 2009, when questions raised about the authenticity of some Motherwells reached the F.B.I. An increasingly bitter controversy erased a fortune’s worth of art, divided longtime professional relationships and cast shadows on the reputations of art experts who vouched for the works. Lawsuits, brought by angry buyers who include a fashion director and a Kuwaiti sheikha, are demanding that the dealers hand over millions of dollars in reimbursement and damages.

Whether Ms. Rosales has begun cooperating with the federal authorities since her arrest in May is uncertain. But though the prosecutors handling her case argued then that she posed “a substantial flight risk” and that no bail conditions could assure her return to court, persuading a judge to detain her without bail, after the new indictment was handed up this week, the prosecutors did not oppose her release on a $2.5 million bond. Steven R. Kartagener, a lawyer for Ms. Rosales, declined to comment on the case.

As for Mr. Qian, his shabby house was empty on Friday. He and his wife, Qiu Yue Xu, had left suddenly a few months ago, neighbors said. Though it was not unusual — he typically spent about half the year in China — this time seemed different. In the yard where once Mr. Qian meticulously painted and repainted a giant portrait of a geisha, newspapers and cats drifted. Several undergarments had been left hung out to dry. Piled wooden canvas stretchers could be glimpsed through a window of the garage; the doors had been left unlocked.

And in nearly every window of the two-story home, plywood or dark paper blocked out the light.