The Isabella Stewart Gardner Museum is pictured in Boston, Massachusetts March 18, 2013. 13 pieces of art were stolen from the Museum in 1990. Jessica Rinaldi/REUTERS

The FBI may have lost their last chance at solving a 27-year-old art heist, according to the Boston Globe.

Two thieves posing as police officers responding to an emergency entered the Isabella Stewart Gardner museum early in the morning March 18, 1990. The thieves then tied up the guards and proceeded to steal 13 works of art valued at over $500 million. It was the largest art heist ever, according to the New York Times, and it all happened in less than an hour and a half.

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Despite the massive haul, a puzzling aspect of the heist was that the thieves walked by and left behind even more valuable work.

The pieces stolen included several paintings by famous French painters Edgar Degas and Édouard Manet as well as sketches and paintings by Dutch masters Rembrandt van Rijn and Johannes Vermeer. Among the stolen art was Rembrandt’s only known seascape, which was titled “Storm on the Sea of Galilee.”

The case was still unsolved Monday, and the art remained missing. Also missing were duct tape and handcuffs the FBI collected at the scene of the crime, the Boston Globe reported. DNA analysis was far more advanced in 2017 than it was in 1990. Kristen Setera, a spokesperson for the FBI, told the Globe that some of the museum evidence was DNA-tested in 2010, but would not specify which items.

The board of trustees of the museum in May doubled the reward for the stolen art to $10 million until the end of the year. The initial reward was $1 million which went up to $5 million in 1997.

“It is our fervent hope that by increasing the reward, our resolve is clear that we want the safe return of the works to their rightful place and back in public view,” said the president of the museum’s board Steve Kidder in a statement.

Boston’s biggest mystery has been the subject of many theories, and the FBI has spent countless hours and resources on the crime. One of the first theories was the caper was pulled off by famed Boston crime boss Whitey Bulger, but authorities have dismissed him as a suspect. Several other New England gangsters have been connected to the heist, but no arrests were made.

Another theory posited that the security guard that let the thieves in was part of the heist. At the time, 23-year-old Richard Abath — a music school dropout — admitted to doing other things against the museum’s security procedures such as showing up to work drunk or high, and once allowed in some friends after hours to have a New Year’s Eve party.

“I operate in the realm of hope,” Anthony Amore, the museum’s security director, told the Globe. “We are never going to stop looking for these paintings.”

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The FBI believed the art traveled around organized crime circles near Philadelphia and was transported to the Connecticut area as well, but the trail has been absolutely cold since 2003.

The museum left the empty frames up as a reminder of what was once there.