NEW YORK, NY.- In the predawn hours of March 18, 1990, following a festive St. Patrick’s Day in Boston, two men dressed as police officers walked into the Isabella Stewart Gardner Museum and walked off with an estimated $500 million in art treasures. Despite efforts by local police, federal agents, amateur sleuths and not a few journalists, no one has found any of the 13 works lost in the largest art theft in history, including a rare Johannes Vermeer and three precious Rembrandts.

The legacy of the heist is always apparent to museum visitors who, decades later, still confront vacant frames on the gallery walls where paintings once hung. They are kept there as a reminder of loss, museum officials say, and in the hope that the works may eventually return. Last month, Richard Abath, the night watchman who mistakenly allowed in the thieves, died at 57. He was a vital figure in an investigation that remains active, but where the trails have grown cold.

Here are five oddities that make this one of the most compelling of American crimes.

The thieves took a really strange array of stuff.

Important paintings were taken from their frames during the heist. But other items that were stolen were not nearly of the same caliber: a nondescript Chinese metal vase; a fairly ordinary bronze eagle from atop a flagpole; and five minor sketches by Edgar Degas. The thieves walked past paintings and jade figurines worth millions, including a drawing by Michelangelo, yet they spent some of their 81 minutes inside fussing to free the vase from a tricky locking mechanism.

The handcuffed guard was later scrutinized.

Abath, one of two guards on duty, was handcuffed and gagged with duct tape. He was never named a suspect. But over the years investigators continued to review his behavior because he had, against protocol, opened the museum door to the thieves. (The second guard, who is still living, was never a focus of investigative interest.) The FBI monitored Abath’s assets for decades but never saw any suspicious income. He consistently said he told investigators everything he knew, and an FBI polygraph he voluntarily took was deemed “inconclusive.”

The empty frames have stayed on the walls.

The museum was once Gardner’s home and she wanted to ensure that her expansive art collection was displayed in the same manner she had arranged it. She stipulated in her will that not a thing was to be removed or rearranged, or the collection should be shipped to Paris for auction, with the money going to Harvard University. Although it has long been reported that the empty frames are left hanging to accord with that will, the museum says that is actually a long uncorrected mistake. “We have chosen to display them,” it said in a statement “because 1.) we remain confident that the works will someday return to their rightful place in the galleries; and 2.) they are a poignant reminder of the loss to the public of these unique works.”

The thieves left behind a prized Rembrandt.

A self-portrait of Rembrandt at 23 was taken down by the thieves but left leaning against a cabinet. “I really believe they probably forgot it,” said Anthony Amore, the museum’s current security chief. The work was on an oak panel, making it heavier than the paintings on canvas that they stole. But it was about the same dimensions as Govaert Flinck’s “Landscape With an Obelisk,” which was also on oak, and stolen.

The list of suspects has been a dizzying stew.

Investigators have looked at all manner of burglars and art thieves and dismissed all sorts of theories. Did Whitey Bulger steal the art to help the Irish Republican Army raise money for arms? No. Did the Mafia want a bargaining chip to help free a member from prison? Maybe. In 2015 the FBI named two long-dead, Boston-area criminals, George Reissfelder and Lenny DiMuzio, as the likely bandits. They have never publicly discussed why.

Investigators still hope to recover the art. The museum upped its reward to $10 million in 2017 from $5 million in 1997 and $1 million in 1990. It has devoted several sections of its website to educating the public about the crime. It embraces publicity in the hope that someone, someday, somewhere will recognize one of the artworks and contact it.

“We have followed every lead and continue to check out new leads,” Amore said, adding, “All that matters is finding out where they are today and getting them back.”

This article originally appeared in The New York Times.