NEW YORK, NY.- Richard Abath, a night watchman whose decision to allow two thieves disguised as Boston police officers into the Isabella Stewart Gardner Museum in 1990 enabled the greatest art heist in history, and one that remains unsolved, died Feb. 23 at his home in Brattleboro, Vermont. He was 57.

His lawyer, George Gormley, confirmed the death but did not provide a cause.

The Gardner Museum, in Boston’s Fenway neighborhood, is one of the country’s leading private art museums, with its eponymous owner’s vast collection of paintings, sculptures and historical artifacts.

Abath was not a professional guard: At a time when museums were significantly more lax with their security, he was a recent music school dropout who took the job to help with bills while focusing on his band, a Grateful Dead-inspired outfit called Ukiah.

By his own admission, he occasionally came to the museum drunk or high, and he said that he once allowed some of his friends into the museum after hours for a party.

The heist took place around 1 a.m. March 18, 1990, the day after the beer-soaked revelries of St. Patrick’s Day. Abath was at the museum’s front security desk; he insisted he was sober.

The other guard on duty had just gone to make the rounds of the museum’s galleries when the two men came to the door, identifying themselves as members of the Boston Police Department and saying they were there to investigate reports of a disturbance. Abath let the thieves into the museum’s vestibule.

“There they stood, two of Boston’s finest waving at me through the glass,” he wrote in an unpublished memoir about the robbery, portions of which appeared in The Boston Globe. “Hats, coats, badges, they looked like cops.”

One of the men asked Abath to come out from behind the desk so they could see if he matched the description of a suspect. As soon as he did, they made him face the wall and handcuffed him.

He quickly realized something was amiss; the men had not frisked him. And he was now several feet away from the museum’s only panic button, back at the desk.

When the other guard returned, the men handcuffed him, too. Then, they covered the guards’ eyes with duct tape and tied them up in different parts of the basement.

Over the next hour and a half, the thieves stole more than a dozen works of art, including pieces by Edgar Degas, Rembrandt van Rijn, Édouard Manet and Peter Paul Rubens, cutting the works from their innate wooden frames. They also took an ancient Chinese beaker and a bronze eagle finial from a Napoleonic-era flagpole.

The men left several valuable works, raising questions about their level of aesthetic sophistication. Still, as thieves, they knew what they were doing: They took several tapes from the museum’s security cameras that would have shown them at work in the galleries.

All together, they took about $500 million in art, the equivalent of $1.2 billion today, making it by far the biggest art heist in history.

Suspicion immediately turned to Abath. City and federal investigators zeroed in on important details, including the coincidence of the thieves arriving so soon after the second guard left to make the rounds. A video camera outside the museum showed Abath briefly opening a side door not long before the robbery occurred.

Abath maintained his innocence throughout his life, and he was never named as an official suspect. He said that he regularly opened the side door to make sure it was locked and that although museum protocol forbade him from letting anyone in after hours, there was no contingency should the visitors be uniformed police officers.

“You know, most of the guards were either older or they were college students,” he told National Public Radio in 2015. “Nobody there was capable of dealing with actual criminals.”

Richard Edward Abath was born May 24, 1966, in Wilmington, Delaware. His father, Walter Abath, was an engineer for Dow, and his mother, Madeline (McKenna) Abath, was a librarian.

Abath attended the Berklee College of Music in Boston, but he left before completing his degree.

He married Diana Hampton in 2006. She survives him, along with his sister, Kathy Buterbaugh; his brother, Jim Abath; and two children from a previous relationship.

He moved to Vermont in 1999, and received a bachelor’s degree from Union Institute & University, an online institution based in Cincinnati. He later worked as a teacher’s aide at a public school.

Abath tried to stay out of the spotlight after the heist, but occasional developments in the case would bring renewed scrutiny about his role.

In 2015, the FBI released security footage from the night of the robbery. It showed a car pull up to the museum and a man in an upturned collar approach the front door. Abath let him in.

The news media and law enforcement touted the tapes as a major turn in the case, and Abath, who had since moved to Vermont, was again interviewed by authorities. But the mysterious visitor turned out to be the museum’s deputy director of security.

“I don’t want to be remembered for this alone,” he told NPR. “But they’re saying it’s half-a-billion worth of artwork. And ultimately, I’m the one who made the decision to buzz them in. It’s the kind of thing most people don’t have to learn to cope with. It’s like doing penance. It’s always there.”

This article originally appeared in The New York Times.