The $500m (£330m) art heist at Boston's Isabella Stewart Gardner Museum was the largest property crime in US history - and still remains unsolved over two decades later.
But on the 23rd anniversary of the theft, detectives said they are finally closing in on the gang and had identified the culprits.
As they announced their breakthrough, federal agents launched a fresh appeal for help in tracking down the 13 stolen works of art, including a rare Rembrandt self portrait, Vermeer paintings and a Degas sketch.
FBI special agent Richard DesLauriers revealed that a $5m reward was on offer by the museum for information leading to recovery of the paintings.
"The FBI believes with a high degree of confidence [that] the art was transported to Connecticut and the Philadelphia region and some of the art was taken to Philly where it was offered for sale by those responsible for the theft," DesLauriers said.
After an attempted sale, the FBI lost track of the contraband and the trail went cold until recently.
DesLauriers said: "We have identified the thieves, who are members of a criminal organisation with a base in the mid-Atlantic states and New England."
"We haven't identified where they are and that's why we are coming to the public for help," said Geoff Kelly, the FBI special agent spearheading the investigation out of the Boston office.
Attorney for Massachusetts Carmen Ortiz said that an offer of immunity might be on the table. Although the statute of limitations for the robbery has passed, Ortiz said thieves could face potential criminal liability charges for possessing stolen works.
Anthony Amore, the museum's chief of security, said: "You don't have to hand us the paintings to be eligible for the reward."
The theft happened in 1990 when two men dressed as police officers gained access to the museum, overpowered and tied up security guards and stole 13 works of art. The Vermeer they stole was one of only 36 in existence.
The FBI has spent two decades following leads around the world.
"The successful return of the paintings to the Gardner Museum would be the final chapter in one of the most significant art theft cases in the FBI's history," said DesLauriers. "And it is a result we would all welcome, seeing these paintings returned to their rightful home."
He refused to reveal the identities of the thieves for fear of hampering the investigation.