BOSTON (AFP) - It's the largest property crime in US history: the three-decade-old theft of Rembrandt and Vermeer masterpieces from a Boston museum by thieves disguised as police officers in the dead of night.

But as the clock ticks toward midnight on New Year's Eve, one detective sits patiently by the telephone and computer screen: could the next call or e-mail finally lead to their recovery and the payout of a US$10 million (S$13.4 million) reward?

"It's hard to be confident. I'm very hopeful," said Anthony Amore, director of security at the Isabella Stewart Gardner Museum, who worked previously for Homeland Security and likens his role to that of a private detective. "100 per cent of our focus is following up on leads we have received."

In May, the museum temporarily doubled to US$10 million a long-standing reward for information leading to the recovery of all 13 works in good condition, hoping that a deadline of midnight on December 31, 2017 would concentrate minds.

It is, in the words of the museum, the "biggest private reward ever offered" for stolen property, and backed by the institution and its board of directors. In the final countdown to the deadline, US press attention sparked an uptick in calls.

"Attention is really snowballing," Amore told AFP. "A lot of calls and e-mails have been coming in."

In perhaps the world's biggest unsolved art theft, the thieves walked into the museum in the early hours of March 18, 1990 and stole 13 works of art in 81 minutes, after handcuffing and tying up two security guards in the basement. The stolen art includes three Rembrandts, a Vermeer, and five sketches and watercolours by Degas, together estimated to be worth more than half a billion dollars.

Last month's record-breaking auction of Leonardo da Vinci's Salvator Mundi for US$450 million in New York has probably made the missing masterpieces only more valuable. Investigators worked tirelessly, but the artwork has never been recovered.

Isabella Stewart Gardner, the wealthy collector who endowed the museum, stipulated in her will that nothing should be changed. To that end, the empty frames of the pilfered art still hang in the same spots.

In March 2013, the FBI identified the thieves publicly as a criminal organisation based in the mid-Atlantic states and New England. But the statute of limitations ran out in 1995, meaning they can no longer be prosecuted. The FBI in Boston urges anyone with information to come forward as soon as possible.

"The investigation has had many twists and turns, promising leads and dead ends," spokesman Kristen Setera said in a statement. "The investigation has led to Europe and Asia. There is no part of the world the FBI has not scoured following up on credible leads." As millions of Americans took time off over the holiday season, Amore stayed wedded to the job in hand. "What's time off?" he quipped. "We have paintings to find." But do rewards even work? Have the paintings not been damaged in the intervening years?

"In my experience they do work, and this US$10 million reward is certainly a significant incentive for anyone with even an inkling of the whereabouts of the pictures to come forward," says Christopher Marinello, a world expert in recovering stolen art. "I still believe, and many people believe these will resurface one day," he told AFP.

If experts were to hazard a guess, they would say the art is still in the wider Boston area. Stashed in the attic, hidden in the basement or shoved under a bed. "You can't hang it on the wall," said Amore. "They are the true definition of pricelessness," he added. "They can never be sold or moved."

While anyone found in possession could still be prosecuted for criminal liability, prosecutors have previously touted potential immunity as bait. "Our focus is not on prosecution. It's totally on recovery," said Amore.

But while the reward led to calls, he is tight-lipped on the potential significance of any leads. Only "a few of them," he said, had been "very valuable". And while the US$10 million reward officially reverts to US$5 million on January 1, Amore refused to be drawn on whether it could be extended. "I'm really not thinking about post-deadline," he said.