For years, investigators discounted the hapless Abath’s role in the unsolved crime, figuring his excessive drinking and pot smoking contributed to his disastrous decision to let in the robbers, who were dressed as police officers. Even if the duo had been real cops, watchmen weren’t supposed to admit anyone who showed up uninvited at 1:24 a.m.

But, after 23 years of pursuing dead ends, including a disappointing search of an alleged mobster’s home last year, investigators are focusing on intriguing evidence that suggests the former night watchman might have been in on the crime all along — or at least knows more about it than he has admitted.

Why, they ask, were Abath’s footsteps the only ones picked up on motion detectors in a first floor gallery where one of the stolen paintings, by French impressionist Edouard Manet, was taken? And why did he open the side entrance to the museum minutes before the robbers rang the buzzer to get in? Was he signaling to them that he was prepared for the robbery to begin?

No one publicly calls Abath a suspect, but federal prosecutors grilled him on these issues last fall. And one former prosecutor in the case has written a recently published novel about the Gardner heist in which the night watchman let the thieves into the museum to pay off a large cocaine debt.

“The more I learn about Rick, the more disappointed I get in him,” said Lyle W. Grindle, the former director of security at the Gardner who hired Abath in 1988.

Now, for the first time, Abath is discussing publicly what happened and admitting that some of his actions are hard to explain, but insisting he had nothing to do with what is regarded as the biggest art heist ever.

Abath, then a rock musician moonlighting as a security guard, said he opened the doors that night because he was intimidated by men dressed as police officers who claimed to be investigating a disturbance. His own uniform untucked and wearing a cowboy hat, Abath knew he looked more like a suspect than a guard.

“There they stood, two of Boston’s finest waving at me through the glass. Hats, coats, badges, they looked like cops,” Abath wrote in a manuscript on the robbery that he shared with The Globe. “I buzzed them into the museum.”

Abath, now 46 and working as a teacher’s aide in Vermont, pointed out that his explanation passed two lie detector tests right after the crime. However, he admits he can’t explain why motion sensors in the gallery that housed the Manet detected footsteps only at the two times Abath said he was in the room — and not later when Abath was bound in the basement and the thieves were looting other galleries.

“I totally get it. I understand how suspicious it all is,” said Abath in a recent interview. “But I don’t understand why [investigators] think . . . I should know an alternative theory as to what happened or why it did happen.”

Now that FBI agents have captured elusive mobster James “Whitey” Bulger, the fate of the Gardner’s stolen masterpieces by Rembrandt, Vermeer, Degas and Manet has replaced “where’s Whitey?” as Boston’s most enduring mystery.

No one has ever been charged in the crime and seemingly promising leads, like the one that led to the search of alleged mobster Robert Gentile’s Connecticut home last May, have invariably fizzled. With no sign of the art works, investigators are left to wonder if the thieves died and took their secret to the grave, or if they are in prison and unwilling to cooperate out of fear of retribution by other conspirators.

But US Attorney Carmen M. Ortiz said the investigation — carried out by her office, the FBI, and Gardner security director Anthony Amore — remains “active, and, at times, fast-moving” even though the statute of limitations for prosecuting the robbery ran out in 1995. Ortiz could still charge anyone possessing the stolen paintings, but she said her office would consider immunity in return for help recovering the masterpieces.

“I am optimistic, and in fact everyone involved in this investigation is optimistic, that one day soon those paintings will be returned to their rightful place in the Fenway,” said Ortiz in a statement.