He and his grandmother, whose third husband was Vincent Astor, the son of financial mogul and Titanic victim John Jacob Astor IV, grew close over the years through a shared love of history and art. (Philip Marshall is a professor of historic preservation and has held faculty positions at Roger Williams University, Columbia University and the University of Vermont; he’s also acted as a consultant for the Hopi tribe in Arizona and worked with the U.S. Tibetan community.)

He had no idea where his path would lead when he first began questioning the actions of his father. “I didn’t know anything about estate planning and estate settlement or elder justice or undue influence in the beginning, but in 2004 I found my grandmother had changed her will. And then I heard things of hers were being sold and I realized these were red flags that something was wrong,” he says.

A turning point came when one of Astor’s most beloved paintings, Childe Hassam’s “Flags, Fifth Avenue,” went missing from the library of her luxurious Park Avenue home. She had promised the painting on numerous occasions to New York’s Metropolitan Museum of Art, but Anthony Marshall had sold it for $10 million and soon afterward bought a home in New Jersey.

Astor also made it clear throughout her life that much of her fortune should go to charity. But as she aged, she became frail and was eventually diagnosed as having Alzheimer’s. As Marshall was to learn, she was coming under the influence of her son, who had power of attorney for his mother, which he used as a “sword and a shield,” says Philip Marshall.

Anthony Marshall shut his mother’s 65-acre Hudson River estate Holly Hill, where Astor had wanted to live out her life in peace. He was accused of doing petty things (like separating Astor from her beloved dachshunds) and dangerous things (refusing to buy her medication), supposedly prompted by his third wife, Charlene (who was not charged).

“It would have taken comparatively little to take care of my grandmother’s needs,” Philip Marshall says. “My father was going to have tens of millions of dollars eventually, even with the large sums going to charity.” But he couldn’t wait. “Charlene may have been behind some of this.”

The red flags kept popping up between 2002 and 2006 as Marshall watched the situation and talked to staff members, who confided in him. But like many in his situation, he did not know what to do at first. His relationship with his standoffish, authoritarian father had never been good, so he could not talk to him. Eventually, he turned for help to a college friend who advised him to “follow his heart and then follow the money.”

With the help of financier David Rockefeller and Annette de la Renta, wife of fashion designer Oscar de la Renta, longtime friends of Astor, Marshall filed a petition for guardianship of Astor in 2006, a role Annette de la Renta took on along with J.P. Morgan Chase. It was supposed to be a quiet, private move, but news leaked out and the front page of the New York Daily News blared “Disaster for Mrs. Astor,” and everything from that point forward was done in public.

Anthony Marshall in 2006 agreed to a settlement of the millions of dollars in dispute in the civil matter without admitting any wrongdoing, but by then it was too late. The Manhattan district attorney’s office had gotten wind of the situation, saw possible criminal activity, and filed criminal fraud and forgery charges against Anthony Marshall and one of Astor’s lawyers.

Astor lived out her final days at her reopened Hudson River estate with her dog, Girlsie. She died in 2007 at the age of 105.