It’s been over 25 years since the greatest art heist in modern history, and the whereabouts of the 13 objects valued at over $500 million stolen from the Isabella Stewart Gardner Museum remain unknown. Yesterday, the institution’s board of trustees announced that it was doubling its reward, from $5 to $10 million, for the return of the works stolen by two men dressed as Boston police officers in the early morning hours of March 18, 1990. The reward money is available immediately, and through December 31, 2017.

The 13 works stolen that morning include Rembrandt’s only known seascape, “Christ in the Storm on the Sea of Galilee” (1633); “The Concert” (ca 1665), one of only 36 known Vermeer paintings in the world; Edouard Manet’s “Chez Tortoni” (ca 1875); and five works by Edgar Degas. Per the terms of the will of Isabella Stewart Gardner, who acquired many masterpieces for her private museum, their empty frames remain on display in the museum. In 1997, the institution increased its initial reward of $1 million to $5 million, and now, 20 years later, it has doubled it in hopes of finally bringing the works home.

“To my knowledge, the $5 million reward is the nation’s largest private reward,” Anthony Amore, the museum’s director of security, told the Boston Globe. “Doubling it to $10 million is a testament to our commitment to bringing these pieces home to their rightful place.” The statute of limitations on the initial theft expired five years later, in 1995, so the two thieves and any accomplices they may have had are no longer at risk of prosecution.

Though there have been occasional false starts on new leads in the FBI’s investigation — including, just this week, a man in West Virginia who claimed to be offering two of the stolen works for sale on Craigslist — its focus has been on figures in the Boston mob. It has most aggressively pursued Robert Gentile, an 80-year-old Connecticut man with ties to organized crime whom federal investigators believe has knowledge of the art’s whereabouts — a claim he has persistently denied. Last month, Gentile pleaded guilty to gun charges and again asserted his ignorance, telling the attorney who had sought to use the charges as leverage for information about the Gardner heist: “You should feel sorry for what you’re doing to me and my wife.”

In his 2015 book Master Thieves, longtime Boston Globe investigative reporter Stephen Kurkjian posited that the art had been stolen to be used as a bargaining chip to secure the release of a jailed mobster. With that goal almost certainly moot by now, he suggested that the art was quite likely hiding in plain sight and that a widespread public campaign appealing to Bostonians’ civic pride might be the best way to secure its return. “There needed to be an effort to show the people of Boston, if not art lovers everywhere,” he wrote, “that what was stolen in that March 1990 heist was as much our loss as it was the museum’s.” Perhaps a $10 million reward is just the sort of public incentive that will bring the works out of the shadows.