THE HAGUE, Netherlands (AP) — A Dutch art sleuth whose search for purloined paintings and sculptures has led to Ukrainian militiamen and Nazi memorabilia collectors has joined the hunt for what he calls the "Holy Grail": A collection worth $500 million that represents the largest art heist in U.S. history.

Arthur Brand thinks a decision last month to double the reward for information leading to the return of 13 works taken in 1990 from Boston's Isabella Stewart Gardner Museum could, indeed should, prompt whoever has the long-sought pieces to return them.

The $10 million reward is on offer only until the end of the year, when it will likely revert back to $5 million.

"All the lights are on green," Brand said in a telephone interview. "If the people do not bring them back this year, it's now or never."

The stunning theft was remarkably simple. Two men masqueraded as Boston police and got into the museum by telling a security guard they were responding to a disturbance.

Once inside, the thieves handcuffed two guards on duty and put them in the museum's basement before snatching masterpieces that included paintings by Dutch masters Rembrandt van Rijn and Johannes Vermeer and French impressionist Edouard Manet.

Investigators have followed an array of leads and suspects — mobsters, Irish gunrunners, local thieves and even a Hollywood screenwriter.

The FBI told The Associated Press in 2015 that two suspects — Boston criminals with ties to organized crime — were dead, but the deaths did not end the search for the Gardner's stolen art. The FBI said investigators believe the collection moved through organized crime circles to Connecticut and Philadelphia, but its exact whereabouts remains a mystery.

The missing pieces include Rembrandt's only known seascape, "Christ in the Storm on the Sea of Galilee," and his "A Lady and Gentleman in Black;" Manet's "Chez Tortoni" and Vermeer's "The Concert," one of fewer than 40 paintings known by the 17th century Dutch painter.

Brand says he is following two possible leads as he seeks to track them down.

Brand, who runs a Dutch agency that helps track the provenance of works of art and advises buyers on the authenticity of works, has a record of successes.

He helped German police seize a huge stash of art in 2015 that included two bronze horse sculptures crafted for Adolf Hitler. Brand also helped recover art stolen from a Dutch museum that had ended up with a militia in Ukraine.

Neither of his leads in the Boston heist is new, but the tenacious sleuth hopes the bigger reward will help.

One focuses on a Dutch criminal who was reportedly in possession of photos of the stolen art and tried to sell the works in the Netherlands and the Belgian city of Antwerp in the early 1990s.

Brand has not seen the photos, but says sources tell him they were taken after the theft. He declined to identify the criminal involved, or his sources.

The lead sounds old, "but if he can tell us who gave ... him these pictures at the time we could trace it back," Brand said.

The other lead is one that U.S. law enforcement authorities have followed and discounted: That a former member or members of the Irish Republican Army, which was responsible for a 27-year campaign of violence in Ireland and the United Kingdom, may have information about the works.

"We are talking with some people about getting more information and trying to make a deal," Brand said, again refusing to elaborate.

Anthony Amore, the Isabella Stewart Gardner's director of security, says the FBI already has pursued the leads Brand is following and they are considered dead ends.

"We've explored the leads Arthur is discussing extensively in the past, and we're confident that we closed them without further need for investigation," Amore said.

He added, "There's never been any evidence presented to us of any value that the art left the United States."

Brand says he and other experts haven't given up on the Irish angle.

"We all think we have very good leads in Ireland, but we still didn't see the paintings, so you never know for sure," he said.

The possibility that whoever now has the art may not face prosecution could also help, along with the huge reward, to get the art back to the Boston museum.

The five-year statute of limitations on crimes associated with the actual theft expired more than 20 years ago, so the thieves can no longer be prosecuted.

Federal prosecutors in Boston have not offered blanket immunity for whoever has the paintings now. They are willing to consider immunity for anyone who can help them recover the stolen works.

"At this point, our primary focus is to get the paintings back," Acting U.S. Attorney William Weinreb told The Associated Press.

That could be enough of an incentive to make Brand the middle man who brings the works back to the museum. Asked to rank his chances of success on a scale of one to 10.

"If you have something in your house, even if it's stolen, and they offer you $10 million and immunity and anonymity, who will hold you back?" Brand said when asked to rank his chances of success on a scale of one to 10. "So in a logical world, I would say 10. But the art world is not logical."

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Associated Press writer Denise Lavoie in Boston contributed.