In this Monday, March 18, 2013 copy photo of a poster provided by the FBI, a likeness of the stolen Rembrandt painting The Storm on the Sea of Galilee is seen at FBI headquarters in Boston. The FBI believes it knows the identities of the thieves who stole art valued at up to $500 million from Boston's Isabella Stewart Gardner Museum more than two decades ago. (AP Photo/FBI)

In this Monday, March 18, 2013 copy photo of a poster provided by the FBI, a likeness of the stolen Vermeer painting The Concert is seen at FBI headquarters in Boston. The FBI believes it knows the identities of the thieves who stole art valued at up to $500 million from Boston's Isabella Stewart Gardner Museum more than two decades ago. (AP Photo/FBI)

BOSTON (AP) — Just after midnight on March 18, 1990, two men posing as police officers pulled off the single largest property heist in U.S. history, stealing 13 pieces of artwork worth as much as $500 million. For more than two decades, the FBI has chased leads around the globe. Now agents believe they know who it was.

But they still don't know where the art is, and they're asking for help.

On Monday, the 23rd anniversary of the theft from Boston's Isabella Stewart Gardner Museum, authorities announced a new campaign aimed at generating tips. Their focus has shifted from catching the thieves to bringing home the precious artwork, which includes paintings by Rembrandt, Manet, Degas and Vermeer.

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

The FBI's Richard DesLauriers says the agency believes the thieves belonged to a criminal organization based in New England and the mid-Atlantic states. He said authorities believe the art was taken toConnecticut 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 artwork, DesLauriers said.

DesLauriers repeatedly rebuffed questions from reporters on the identities of the thieves, saying releasing their names could hamper the investigation. He refused to say whether the thieves are now in prison on other charges, or whether they are dead or alive.

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

Gentile, 76, of Manchester, Conn., was not charged in 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 knows nothing about the heist but was acquainted with people federal authorities believe may have been involved.

The FBI also searched the Worcester home of an ex-convict who has a history of art theft.

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.

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

The new publicity campaign will include a dedicated FBI website on the theft, http://www.FBI.gov/gardner, video postings on FBI social media sites and digital billboards in Connecticut and Philadelphia.

DesLauriers said authorities believe someone not involved in the theft has seen the artwork without realizing it is stolen.

"It's likely over the years that someone — a friend, neighbor or relative — has seen the art hanging on a wall, placed above a mantle or stored in an attic. We want that person to call us," DesLauriers said.

The FBI said it is re-emphasizing a $5 million reward being offered by the museum for information that leads directly to the recovery of the art.