Ann Freedman had come to Knoedler one last time.

On a mid-February day, she approached the mansion at 19 East 70th Street, where New York’s most venerable art gallery used to be, before its sudden, shocking closing last fall amid forgery allegations. “It’s amazing to think that this institution never stopped for 165 years,” she said. “It didn’t stop during the Civil War, World War I, World War II … I kept it open on 9/11.”

Now the doors were locked, the building cleaned out. The new owner was about to take possession. Knoedler’s former director had wangled a walk-through: a chance, as she put it, to be the last one in and the last one out of this gallery that had once sold Raphaels and Vermeers to Mellons and Fricks. She seemed not to wonder whether she was part of the reason these rooms were now empty.

Freedman is 63 now; tall and gaunt, with silver corkscrew curls and round wire-rimmed glasses, she is given to stylish pantsuits and dramatic belts. She’s genial but somehow remote, the sort who seems to talk mostly to control the airspace.

“The significance of this institution,” she declared, “will not rest on the David Herbert collection.”

But it will.

The e-mail that brought the art world’s latest scandal to light came to Knoedler last November 29. It disclosed the results of forensic tests done to a Jackson Pollock painting, Untitled 1950, that the gallery had sold in 2007 for $17 million to Pierre Lagrange, a London hedge-fund multi-millionaire. Done in the painter’s classic drip-and-splash style and signed “J. Pollock,” the modest-size painting (15 inches by 281 1/2 inches) was found to contain yellow paint pigments not commercially available until about 1970. This was discouraging, since the painter’s fateful car crash had occurred on August 11, 1956.

Lagrange wasn’t just discouraged. He was furious. Fiftyish, given to long brown locks and blue jeans, he had startled London society in 2011 by leaving his wife and three children, only to take up with a 42-year-old male fashion designer named Roubi L’Roubi. Selling the painting had been part of his effort to divide assets in connection with a divorce settlement that might be one of the largest in British history. Now he was giving Knoedler 48 hours to agree to reimburse him or face a lawsuit. To the art world’s astonishment, the venerable gallery simply pulled its brass doors shut. (Knoedler has said that the closing was a business decision unrelated to the Lagrange suit.)

The Pollock turns out to be just one of roughly 20 paintings with the same sketchy backstory, sold for tens of millions by Knoedler on Freedman’s watch: the so-called David Herbert collection. All are purportedly by the giants of 20th-century Abstract Expressionist painting: Willem de Kooning, Mark Rothko, Barnett Newman, Clyfford Still, Franz Kline, in addition to Pollock. According to Freedman, all came from Glafira Rosales, a Long Island woman virtually unknown in the art world. She claimed to represent an anonymous owner Freedman calls “Mr. X Jr.” Rosales is now a subject of investigations by the F.B.I. and the U.S. Attorney’s Office, which has impaneled a grand jury. And all the works may be fakes, although no one can yet say, with absolute certainty, whether any are: even forensic tests can be flawed. Freedman, whose lawyer claims she’s not a subject of the investigations, is determined not only to salvage her reputation but to prove she has discovered the greatest unknown trove of modern masterpieces. “The works are of a five-star quality,” she says. “Maybe a few are four-star, but mostly five-star, which is why they’ve stirred such attention.”