In the early hours of March 18, 1990, two men in police uniforms walked up to a side entrance of the Isabella Stewart Gardner Museum, a small but swanky art collection in a historic neighborhood of Boston.

“Police! Let us in,” they announced into the intercom. “We heard a disturbance in the courtyard.”

They were buzzed in.

The young security guard on duty was tricked into stepping out from behind his desk and promptly put in handcuffs. “Why are you arresting me?” he demanded.

But it wasn’t an arrest.

“This is a robbery,” he was told, according to an ArtNews account of that night. “Don’t give us any problems and you won’t get hurt.”

By the time the real police arrived hours later, the men were gone, along with 13 works worth an estimated $500 million. Both of the thieves, and the art they stole, have been missing ever since; it’s one of the highest-profile heists in history.

Two and a half decades later, new clues in the case are slowly beginning to trickle out. In 2013, the FBI announced that it knew the identity of the thieves, members of some sort of criminal organization based in the mid-Atlantic and New England (though they didn’t name names). The agency also said that the works had been offered for sale in Philadelphia roughly 10 years earlier in 2003, but it didn’t know what happened to them since.

On Thursday, authorities dropped another bread crumb for art enthusiasts and amateur investigators, who they hope can help with the case: security footage from just after midnight on March 17, 1990 — almost exactly 24 hours before the theft.

The murky, black and white video appears to show what could be a dry run for the next night’s robbery. First, a car matching the description of the automobile from the heist pulls up next to the museum. A man emerges. Then it switches to an indoor camera pointed at the security desk by the side door where the thieves would later enter.

The young security guard who would wind up in handcuffs the next night — not named in the FBI release but identifiable as 23-year-old Richard Abath — opens the door and admits a man wearing a coat with an upturned collar. They appear to talk and fumble through paperwork (only the tops of their heads can be seen) then disappear for several minutes.

Then the man walks out. The car drives away.

In this March 11, 2010, photo, the empty frame from which thieves stole Rembrandt’s “Storm on the Sea of Galilee” remains on display at the Isabella Stewart Gardner Museum in Boston. (Josh Reynolds/Associated Press)

Speaking to the New York Times, FBI officials were tight-lipped about the tape’s origin and significance. They said the footage was pulled immediately after the theft but did not say whether it had been reviewed before 2013, when the FBI appeared to revisit the case with new vigor. Nor did they comment on questions about Abath, “for privacy reasons.”

But, the Times noted, “the release of the video seems to imply . . . that his actions are again being scrutinized as part of the investigation into a case that has bewildered the authorities for a quarter-century.”

Abath, now in his late 40s and living in Vermont, has long denied any involvement in the theft, though he said he knew that admitting unauthorized visitors was against museum policy. According to the Boston Globe, he has previously admitted to bringing friends into the building after hours at least once before the robbery but never said that he let anyone in the day before the heist.

“What you see in the video does not comport with what we have been told in the past,” Anthony Amore, the museum’s director of security, told the Globe.

U.S. Attorney Carmen M. Ortiz told the Times that FBI officials have been examining the footage for two years in an attempt to identify the strange man and his car. Now they’re sharing it with the public in hopes that it might lead to a tip that could unravel the years-old mystery.

The theft that was carried out that night in 1990 was both polished and perplexing.

Abath and a fellow security guard were handcuffed in the museum’s basement, their heads wrapped in duct tape. Then the thieves went upstairs and set about their work.

For the next 81 minutes, the men clumsily and seemingly arbitrarily smashed glass cases and cut paintings from their frames. They made off with a varied and unquestionably valuable haul — three Rembrandts, a Vermeer, a Manet, a Flinck, five drawings by Degas, an eagle ornament from the Napoleonic wars and a 3,100-year-old Shang dynasty bronze beaker. But they inexplicably passed over several more precious works: a sketch by Michelangelo, a painting by Titian.

When they left, they took the museum’s surveillance footage with them — but not the tape from the night before.

“It’s difficult to understand why the thieves took what they did, an eclectic collection,” Geoffrey J. Kelly, an FBI agent who had worked on the Gardner case for many years, told ArtNews in 2009. “They were certainly in the museum long enough to take whatever they wanted. They treated the guards well. That’s professional.”

Down in the basement, Abath — a dropout from the Berklee College of Music and a guitarist in a struggling rock band — says he was singing to keep himself calm. Over and over again he repeated the lyrics to Bob Dylan’s “I Shall Be Released”: “They say everything can be replaced/Yet every distance is not near/So I remember every face/Of every man who put me here.”

He still feels horrible about what happened, he told his wife in a conversation broadcast on NPR earlier this year.

“Ultimately, I’m the one who made that decision to buzz them in,” he said. “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.”

In this image from a March 18, 1990, surveillance video, an unauthorized visitor walks inside the rear entrance of the Isabella Stewart Gardner Museum in Boston. Robbers stole more than a dozen works of art about 24 hours later. (Isabella Stewart Gardner Museum/U.S. Attorney’s Office via Associated Press)