BOSTON (CBS) – Twenty five years ago, thieves pulled off the largest art heist in history. It happened right in the heart of Boston at the Isabella Stewart Gardner Museum. Investigators say they are confident the paintings will be found.

“We have a high degree of confidence that we know who did this,” said Pete Kowenhoven of the FBI in Boston.

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Kowenhoven says the two suspects are now dead. “Two individuals that dressed up like the Boston Police officers are deceased,” he said.

On March 18, 1990, those imposter police officers got buzzed into the museum, tied up the guards, and 81 minutes later, 13 art treasures were gone. The FBI is not naming the thieves.

Sketch of suspects in Gardner heist (WBZ-TV)

Sketch of suspects in Gardner heist (WBZ-TV)

“We know who did it and based on that working our way backwards as to how they did it, who were their associates, we are tracking that currently,” Kowenhoven said.

Twenty five years later the Boston FBI remains in constant contact with museum security.

“I always think to myself the paintings will be back,” said Anthony Amore, the director of museum security.

Those masterpieces are worth more than $500 million and include works by Rembrandt, Degas, Manet and Vermeer. Their museum frames remain empty to this day.

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“We keep our frames on the wall because we want people to have hope that they will come back,” Amore says. “These pieces are vitally important not just to the city but the world.”

Shortly after the heist, according to the New York Times, a stolen Manet was spotted in the apartment of George Reissfelder. The Boston man was released from jail in 1982 after he was wrongfully convicted of murder.

John Kerry with George Reissfelder in 1982 (WBZ-TV)

John Kerry with George Reissfelder in 1982 (WBZ-TV)

He was represented at the time by attorney and now Secretary of State John Kerry. You can see some similarities with the police sketch and Reissfelder, who died in 1991.

Investigators also dug up a Connecticut backyard three years ago, but no paintings were found.

The FBI says they continue to follow up on all leads.

“We are working around the clock on this case,” Kowenhoven says.

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There is a $5 million reward for information leading to safe return of the paintings.

Paula Ebben