In what is perhaps Boston's last enduring mystery, federal investigators last week stirred public excitement by announcing that they know who was behind the notorious Gardner Museum heist 23 years ago, and had traced the stolen art from Boston to Connecticut and then to Philadelphia.
But one central question remains: Where is the art now?
Those dedicated to the recovery of the stolen art — observers and retired investigators alike — worry that if the priceless artworks aren't found now they could be lost forever.
"I think we all have to be realistic — knowing that so many people that we believed had some knowledge of what happened have since died, without saying anything," said James J. McGovern, who worked on the case as a special US attorney in 2005 and 2006 and last year authored "Artful Deception," a novel about the investigation. "My hope is the same as everyone else's who has worked on this case — please, someone who knows something of [the artworks'] whereabouts, step forward."
Over the last 23 years there have been tips, raids, searches, and press conferences. And on each occasion, investigators have come up empty-handed, leading them to embark on a new path.
Even in Philadelphia, where the FBI has extended its investigation into the heist, members of the art community are aghast at the idea that the paintings could have made their way through a city of museums and art schools — where the FBI's art theft unit is based — without anyone noticing.
"I never heard anything about any attempt by the mob to sell the artwork in Philly in 2002, and I was right there then," said Robert Wittman, who was the FBI's lead undercover agent for recovering stolen artwork until his retirement in 2008.
Larry Becker, an established art owner and dealer who runs the Larry Becker Contemporary Art gallery in Philadelphia, said he has heard only tales of the heist. "Hopefully they're not destroyed, and hopefully they can get them back and bring them to the Gardner."
Becker welcomed the announcement last week that the FBI has launched a public awareness campaign in the Philadelphia area to make people aware of the artworks and the plea for their return.
"They could be in a restaurant, someone's home, and someone's not knowledgeable to know they came out of the Gardner," he said. "The more they spread the imagery, the more likelihood that someone could see them."
His wife, Heidi Nivling, who also runs the gallery, added, "The thing we have to remember is there's . . . only one of each painting in the universe, and that's one of the things that makes them valuable."
"I believe most curators, art historians, and artists would say the true value is the history," she said.
The announcement that the FBI was engaging in a public awareness campaign about the heist and the $5 million reward for the return of the artwork is the latest strategy in the enduring search for the stolen art.
The FBI also disclosed that investigators believe they know who stole the paintings, though they would not identify the thieves, saying it would hinder their investigation.
They also believe that the paintings have changed hands several times, making their way through organized crime circles from Boston to Connecticut and Philadelphia, where some of the art was offered for sale as recently as a decade ago.
The course of the investigation appears to confirm a line of inquiry that has centered in large part on a reputed gangster from Connecticut, whose home was searched last year, and his connections to the Philadelphia Mafia.
Meanwhile, at least a dozen members of the Philadelphia Mafia, including reputed boss Joseph "Uncle Joe" Ligambi, have been accused in a federal investigation of racketeering. Observers have speculated on whether anyone under the pressure of a federal indictment might offer information.
Former US attorney Donald Stern, who oversaw the federal investigation for eight years, said the FBI's announcement that it has intensified the investigation and tracked some of the art to Philadelphia could help to generate new leads.
"Anyone who may have control of those paintings or know where they are now has an incentive to make a deal," he said. "It's like a fugitive hiding in the weeds, hearing the bloodhound barking. There may be only a limited time now for him to make a deal."
The Gardner heist remains among the last of Boston's mysteries, following the arrest two years ago of fugitive James "Whitey" Bulger. And officials hope that the public awareness campaign that led to the arrest of Bulger and girlfriend Catherine Greig will have the same success in the Gardner investigation.
Damon Katz, chief counsel for the FBI's Boston division, last week would only say that the agency has been analyzing tips that have come in since the announcement.
"Everything that comes in, we will take a look at," he said. "With the public's cooperation, we do hope to get [the art] back."
The theft was carried out on March 18, 1990, when two men posing as Boston police officers conned their way into the museum and made off with 13 works of art, including three Rembrandts and a Vermeer.
The paintings, which have been valued at $500 million, are essentially priceless, experts say, given their notoriety.
The probe initially centered on a crew from Dorchester with ties to the New England Mafia. From there, however, investigators shifted their focus to a gangster, the late Robert Guarente, who had ties in organized crime in Boston and Connecticut and Philadelphia.
And since 2010, when investigators say they received a tip from a caller that sparked the latest flurry of activity, officials have searched Guarente's old home in Maine and raided the home of one of his former associates in Worcester.
Last May, investigators searched the Connecticut home of one of Guarente's acquaintances, Robert Gentile, who has ties to the Philadelphia Mafia. Investigators recovered no paintings but discovered a list of the paintings and their estimated value.
Gentile has denied knowing the location of the paintings.
Thomas McShane, a former FBI undercover art investigator who has written a book about the heist, said he has followed the investigation since his retirement in the mid-1990s, fascinated with the tale. Monday's announcement has introduced a new intensity, he said.
"This was monumental. . . . They're on the trail," he said. "If they've had this information, they believe they know who these two guys are, I believe they may have something up their sleeve and maybe they want to put more pressure up there."
Milton J. Valencia can be reached at mvalencia
Follow him on Twitter @miltonvalencia.