BOSTON — It’s been called the biggest art heist in U.S. history, perhaps the biggest in the world. But 25 years later, the theft of 13 works from Boston’s Isabella Stewart Gardner Museum remains unsolved.

The theft has spawned books, rumors and speculation about who was responsible – and multiple dead ends.

Yet authorities and museum officials remain hopeful, noting that stolen art almost always gets returned – it just may take a generation or so.

“Although a quarter-century has passed since the art was stolen, we have always been determined to recover it and we remain optimistic that we will,” said Anne Hawley, the Gardner’s director, who was in charge at the time of the theft.

In the early hours of March 18, 1990, two men disguised as police officers talked their way into the museum by saying they were responding to a call. They overpowered two guards, bound them with duct tape and spent 81 minutes taking 13 works of art, including masterpieces by Rembrandt, Vermeer, Degas and Manet.

Authorities say the artwork is worth perhaps as much as a half-billion dollars.

Some of the works, including Rembrandt’s “Storm on the Sea of Galilee,” were cut from their frames that to this day hang empty. “It is our way of remaining hopeful,” museum marketing director Kathy Sharpless said.

The FBI announced two years ago that they think they know the identities of the thieves, but no names have been disclosed or arrests made.

The FBI has chased down thousands of leads around the globe, but the answer may be closer to home.

Richard DesLauriers, former agent in charge of the FBI’s Boston office, said two years ago that investigators believe the thieves belonged to a mob based in New England and the mid-Atlantic. They believe the art was taken to Connecticut and Pennsylvania in the years after the theft and offered for sale in Philadelphia.

“That announcement did generate some good tips but no recovery,” said Geoff Kelly, a member of the FBI’s art crime team.

In 2012, the FBI fruitlessly searched the property of a Connecticut mobster.

The museum is offering a $5 million reward for the return of the artwork “in good condition.” The government’s reward is immunity.

The museum marked the infamous anniversary by launching a virtual tour on its website.