The FBI says it has solved the decades-old mystery of who stole $500m (£330m) worth of art from Boston's Isabella Stewart Gardner Museum, but it is withholding the identities of the thieves, adding a further twist to the largest property heist in US history.

On the 23rd anniversary of the theft, authorities announced a new publicity campaign aimed at generating tips on what they still do not know: where the missing art is. Their focus has shifted from catching the thieves to bringing home the precious work, including paintings by Rembrandt, Manet, Degas and Vermeer.

"The key goal here is to recover those paintings and bring them back," US Attorney Carmen Ortiz said at a news conference at the FBI's Boston headquarters.

Just after midnight on 18 March 1990, two men posing as police officers pulled off the heist, stealing 13 pieces of art in 81 minutes.

For more than two decades, the FBI has chased leads around the globe, finally making progress over the last few years so that they now believe they know the identity of the thieves.

The FBI's Richard DesLauriers says the agency believes the thieves belonged to a criminal organisation based in New England and the mid-Atlantic states. He said authorities believe the art was taken to Connecticut and the Philadelphia region in the years after the theft, and offered for sale in Philadelphia about a decade ago.

After the attempted sale, the FBI does not know what happened to the art, he added.

Ortiz said the statute of limitations had expired on crimes associated with the actual theft. She said anyone who knowingly possessed or concealed the stolen art could still face charges, but said prosecutors were willing to discuss potential immunity deals to get the art back.

DesLauriers repeatedly rebuffed questions from reporters on the identities of the thieves, saying releasing their identities could hamper the continuing investigation. He refused to say whether the thieves were now in prison on other charges, and would not say whether they were dead or alive.

Last year, a federal prosecutor in Connecticut said the FBI believed a Connecticut man Robert Gentile had some involvement with stolen property related to the art heist.

Gentile, 76, of Manchester, Connecticut, was not charged over the heist, but pleaded guilty in November in a weapons and prescription drugs case. Gentile's lawyer A Ryan McGuigan said at the time that Gentile testified before a grand jury investigating the heist. He said Gentile knew nothing about the heist, but was acquainted with people that federal authorities believe may have been involved.

Ortiz said the investigation was "active and at times fast-moving" over the past few years.

In the meantime, empty frames hang on the walls of the museum, a reminder of the "enormous loss" and a symbol of hope that they will be recovered, said Ortiz. The stolen paintings include: The Concert by Johannes Vermeer; and three Rembrandts, A Lady and Gentleman in Black, Self-Portrait and Storm on the Sea of Galilee, his only seascape.