Experts say it's very tough for most thieves to sell well-known artworks.
"Art is a very, very difficult item to liquidate," says Hall. "Most of the time, professional thieves have a buyer before they steal it."
With major, well-known works, like those taken from the Isabella Stewart Gardner Museum in Boston in 1990, any buyer would also have to be willing to keep it totally secret and having it only for his or her private enjoyment.
Kelly says the Hollywood-esque idea of professional thieves stealing art to order is overblown. "More commonly, art theft is more of a crime of opportunity," he says, with those thieves "just as likely to go out and steal a car."
There is also a known pattern among some crooks to steal valuable works of art simply to have something to bargain with if they get caught for other crimes. The idea is that a thief could offer to return the art in exchange for a lighter sentence.
"That does happen… It's a valuable insurance policy," says Kelly. He adds that there's been speculation that sort of "crime insurance" was behind the Gardner heist.
The feds believe they know who the Gardner thieves were. They believe the works by Rembrandts, Vermeer, Degas and other artists moved from Massachusetts to Connecticut to Philadelphia. They also think the artworks have changed hands several times.
One obvious Connecticut connection is Gentile, a 75-year-old reputed gangster with ties to the Philadelphia underworld. His Manchester home and yard were searched by the FBI in 2012 and he's in prison on federal drug and weapons charges.
FBI officials declined to directly name anyone as being part of the conspiracy that resulted in one of the biggest art thefts in history. They say revealing names could frustrate their efforts to recover the stolen art.
Kelly says there were "confirmed sightings of the paintings a little over a decade ago."
The statute of limitations has run out for the Gardner theft, but there is no statute of limitation for possession of stolen property, which makes it real tough to dispose of that stolen art.
At the same time, prosecutors have said they're willing to discuss immunity for anyone who comes forward with information. And there's that $5 million reward hanging out there.
So Kelly doesn't hesitate when asked about the odds of one day finding those lovely works of art that were ripped off the walls of the Gardner museum.
"We're very confident," he says.
Edouard Manet's "Chez Tortoni," another stolen artwork. (Image courtesy Isabella Stewart Gardner Museum)