The buzzer rang at 1:24 a.m. While the last of Boston’s St. Patrick’s Day revelers downed their final drinks before turning in on March 18, 1990, night watchman Richard Abath looked up from his security desk to see two men in police uniforms and caps at the door of the Isabella Stewart Gardner Museum.

“Police,” announced one of the men. “We’re here about the disturbance.”

Although trained to first phone Boston police headquarters to confirm officers’ names and badge numbers, the 23-year-old Abath—an aspiring rock musician who had given his notice days earlier—pressed a button to let the pair inside. The uniformed men ordered Abath to summon his fellow guard, 25-year-old Randy Hestand, to the foyer.

“You look familiar,” one of the men told Abath. “I think we have a warrant out for your arrest. Come out from behind the desk and show us some identification.” Making his second protocol breach, Abath left his post and stepped away from the museum’s only panic alarm. The uniformed men suddenly forced the guard against a wall and handcuffed him before doing the same to the arriving Hestand, who was filling in for a sick colleague and working the night shift for the first time.

“This is a robbery, gentlemen,” announced one of the imposters, as if that wasn’t already clear. Using duct tape, the intruders wrapped both guards like mummies, even covering Abath’s shoulder-length curly hair, and led them to the basement where they handcuffed Hestand to a sink and Abath to a workbench 40 yards away.

The thieves then ransacked the museum that philanthropist Isabella Stewart Gardner opened in 1903 as a benevolent gift to the people of Boston. The operation was hardly surgical. The thieves smashed gilded painting frames onto marble floors and sliced canvasses from their wooden backings. The robbers left an empty frame on the office chair of the museum’s security director and removed the recording tape from the closed-circuit television system before departing 81 minutes after their arrival in a dark-colored hatchback that dissolved into the misty night.

The next morning, Abath and Hestand were found, but missing were 13 artworks worth $500 million. The most expensive piece was Johannes Vermeer’s “The Concert,” one of only 36 known paintings by the Dutch master. The robbers absconded with a sketch and two paintings by Rembrandt, including his only known seascape, “Storm on the Sea of Galilee.”

Also taken were a Govaert Flinck landscape, Edouard Manet’s “Chez Tortoni,” five watercolors and sketches by Edgar Degas, a finial eagle that sat atop a Napoleonic flag that the thieves could not unscrew from the wall and an ancient Chinese vase. Strangely, the robbers had left untouched the museum’s most valuable painting, Titian’s “The Rape of Europa,” but an unwound coat hanger found near the candy machine suggested that they also pilfered chocolate bars.

A day after the break-in, the Boston Globe reported that art experts speculated that the treasures “were probably contracted for in advance by a black-market collector outside the country.” Latin American drug cartels, Irish Republican Army militants and even Vatican operatives were floated as suspects. Suspicion also fell on notorious Boston mobster James “Whitey” Bulger, but he was searching for whoever committed the audacious crime on his home “turf” as well so he could exact a cut. The FBI investigation into Abath, who was the only person tracked by motion sensors in the gallery from which the Manet was swiped, did not yield any answers.

For 25 years, leads continued to prove false, and the case grew cold. “It’s a spectacular mystery—both because of the value of the pieces and the daring of the bad guys,” says Stephen Kurkjian, author of Master Thieves: The Boston Gangsters Who Pulled Off the World’s Greatest Art Heist. He says, however, that the perpetrators left behind clues that a wealthy collector was not the mastermind. “The brutal way they treated the artwork in stealing it belies that this was a score commissioned by a Dr. No or a Mr. Big who couldn’t live without a Manet, Rembrandt or Vermeer.”

Kurkjian believes the theft was hatched in Boston’s underworld, which in 1990 was being rocked by a turf war between one criminal gang led by Frank Salemme and another led by Vincent Ferrara and J.R. Russo. As Kurkjian reports, the FBI informed the museum nine years before the heist that a Mafia gang was plotting to strike with thieves posing as police officers. According to Master Thieves, low-level gangster Louis Royce knew firsthand about the museum’s lax security since he had surreptitiously slept overnight in its galleries as a teenager.

The most likely scenario, according to Kurkjian is that a wheelman for Ferrara carried out the heist. The wheelman, who confessed to the crime according to his boss, had toured the museum several times with notorious art thief Myles Connor Jr. and had been seen with two police uniforms in a bag at a local social club. The motive? To use the pieces as bargaining chips to free Ferrara, who was in jail on racketeering charges. Kurkjian notes that in 1975 Connor received a reduced sentence in exchange for the return of a Rembrandt stolen from Boston’s Museum of Fine Arts. “Out of that came this wild belief that if you steal a piece of art, you will be able to make a deal for a criminal associate,” he says.

Ferrara’s wheelman was murdered in 1991, and if he was the perpetrator, the artwork's location might have gone to his grave. “Even inside the gangs, this would not have been a shared secret,” Kurkjian says. “If the gang members who pulled it off were killed—and that is my sense since they were into all sorts of violent activities—no one may know exactly where they hid the stuff except for rumors and innuendos among former gang members and their families.”

On the heist’s anniversary in 2013, the FBI reported “with a high degree of confidence” that it, too, believed a criminal organization was behind the robbery. However, it never publicly named a suspect. The statute of limitations for the robbery ran out in 1995, and federal authorities have said they are willing to offer immunity for possession of the stolen property if the pieces are returned. A reward is still being offered.

Since Gardner’s 1924 death, her museum has been frozen in time, with her will mandating that no artwork could be moved from the precise location where she had placed it. For more than 30 years, the Dutch Room has been untouched as well with four empty frames hanging from the wall, reminders of the personal loss for art lovers, but also hopeful symbols that the paintings will someday return.