Where does stolen art go?

In a murder investigation, each day that goes by means the trail grows colder and colder. But unlike most other crimes, the more time passes in an art theft, the closer investigators could be getting to cracking it, according to Prouty.

Statistically, most art thefts take several years or even decades to solve. Once a thief pulls off the robbery itself, another challenge arises — selling the work without getting caught. Armed with the knowledge that a recently stolen work could be familiar to even the general public in a highly publicized case, thieves often keep the art for a time before trying to fence it.

And when the art is passed along, authorities often receive tips about who is possesses it.

Special Agent Charles Prouty, head of the FBI's Boston field office

"As time passes, relationships change, people are divorced, people may die," Prouty said.

His office continues to receive tips that are actively investigated.

"We've had close calls," he said.

One tipster thought they saw Rembrandt's "Lady and Gentlemen in Black" in Japan.

Another lead came from a Boston FBI agent who was walking through nearby Charleston, the historic Boston-area town that hosts a Navy Yard and Old Ironsides, when he thought he saw one of the stolen works through a window.

In both instances, however, the paintings were replicas, not originals.

Rembrandt's Lady and Gentleman in Black

Investigators are also on the lookout for works being sold through art catalogs and small dealers.

"People talk about a black market in stolen art," explains Anna Kisluk, director of art services of the Art Loss Register, an international agency funded by the major auction houses to serve as a database for stolen art. "Certainly there's some of that."

But for the most part, she says, much art is recovered through happenstance — stumbling across it in a small antique shop or from dealers who don't even realize the work is stolen.

"Fifty-one percent of stolen art is found in art catalogs. People try to fence it," Prouty explains.

While many art auction houses, including leading ones like Christie's and Sotheby's, make it a policy to check if an art work is stolen before accepting it for auction, there is no requirement to do so by law, making it easy for stolen art to slip through the cracks with smaller establishments.

While Prouty declined to elaborate on specifics citing the active investigation, he says that nothing is being ruled out.

The identity of the two mysterious robbers is still unknown, aside from their descriptions — both white males who wore Boston Police uniforms and fake-looking shiny black mustaches.

The two imposters wore Boston Police uniforms during the March 18, 1990 robbery

According to the FBI's description, one of the suspects appeared to be in his late 20s to mid-30s, 5-foot-7 to 5-foot-10 inches tall with a medium build, dark eyes and short black hair.

The other was in his early to mid-30s, approximately 6-feet tall, between 180 and 200 pounds, and had broad shoulders, dark eyes and medium-length black hair.

But while authorities hope that identifying the two suspects leads them to the art, they might never be prosecuted for the crime.

That's because the statute of limitations on robbery in Massachusetts ran out six years ago, so neither could be prosecuted as long as the scheme was plotted by someone else and they no longer had any of the 13 works in their possession. The only applicable federal law that can be enforced up to 20 years after the crime wasn't enacted until 1994, four years after the Gardner heist.

In fact, the Gardner case spurred passage of the Theft of Major Artwork statute, making it a felony punishable by up to 10 years in prison for stealing any museum art more than a century old or worth at least $100,000.

While the identity of the two mysterious robbers remains unknown, art investigator Harold Smith has his theories.

"If we're honest we don't really know," Smith admits. "But the actual thieves to me sound like down-to-earth burglars."

History shows that most art crimes are not committed for the love of art.

"Most thieves steal for money. They're in it for the buck," concurs Anna Kisluk of the Art Loss Register. "Thieves first and foremost are not art connoisseurs."

Some theories of the Gardner heist are also based on which works the thieves selected.

The museum's Titian Room, where the masterpiece The Rape of Europa is located, went untouched by the thieves

The most valuable piece in the museum, Titian's "The Rape of Europa," for example, was left untouched. Other works that were stolen, such as the finial from a Napoleonic flag and four sketches by Degas, are not worth nearly as much as some of the paintings.

"They're nice, they're interesting," explains Kisluk. "They're not masterpieces."

Some of the more unusual details of the robbery continue to perplex investigators and Smith alike.

One of the Rembrandts was savagely cut out of the frame, probably diminishing its value significantly.

"I can't figure that out," Smith admits.