The FBI's Boston office announced today that it is confident it knows the identities of the thieves who stole 13 works of art from the Isabella Stewart Gardner. The heist, considered one of the most significant in American history, took place 23 years ago today.

"With a high degree of confidence we believe those responsible for the theft were members of a criminal organization with a base in the Mid-Atlantic states and in New England," said Richard DesLauriers, special agent in charge of the FBI’s Boston office.

The FBI is not publicly identifying the thieves, because of the ongoing investigation.

DesLauriers said the FBI has figured out where the art -- including rare paintings by Rembrandt and Vermeer -- was taken after the theft.

"Recently we determined that approximately a decade ago, some of the art was brought to Philadelphia where it was offered for sale,” he said. “However, we don't know where the art is currently located."

DesLauriers is relaunching an international effort to raise public awareness of the heist, and to solicit information. The FBI wants current and past residents of Philadelphia and Connecticut to take a look at online images of the missing paintings, in case they look familiar.

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

The FBI said, based on previous art thefts and recoveries, the stolen paintings and antique objects have likely changed hands several times, and have probably been well cared for. The Isabella Stewart Gardner museum is offering a $5 million reward for the return of the art. U.S. Attorney Carmen Ortiz said the focus is on getting the items back, not necessarily on putting the thieves behind bars.

"In terms of the actual theft, the statute of limitations has run,” Ortiz said. “But there's potential criminal liability for anyone who's in possession, possession of stolen property, or concealing the paintings."

Ortiz explained that in some circumstances, immunity can be offered to an individual in possession of stolen art. And in this case, it's art that has fascinated visitors of the Gardner museum for more than 2 decades. The frames of the missing paintings still hang on the wall, empty.

“Two men posed as Boston Police officers bluffed their way into the museum by telling the night guards that they were investigating a disturbance,” DesLauriers said. “After tying up the security guards in the museum’s basement, the thieves spent 81 minutes seizing works of art now valued at more than $500 million.”