BOSTON (AFP).- The FBI said Monday that they finally know who conducted a daring art heist in Boston exactly 23 years ago -- but the thieves can no longer be prosecuted.
For two decades, the 1990 theft of 13 works of art from the Isabella Stewart Gardner Museum, including rare paintings by Rembrandt and Vermeer, has been one of America's greatest unsolved crimes.
Now, says the FBI's Boston chief Richard DesLauriers, agents "confirmed the identity of those who entered the museum and others associated with the theft."
At a news conference, he touted "significant investigative progress."
But there are two big hitches.
First, the thieves who hit the museum dressed as Boston police officers essentially got away with it -- because the robbery "occurred 23 years ago, the statute of limitations has run" out, US Attorney Carmen Ortiz told reporters.
The prosecutor said the only likely prosecutions were over "criminal liability for anyone in possession."
In addition, the FBI still doesn't know where the masterpieces are hidden.
DesLauriers said it was clear now for the first time that the art had been stolen by "a criminal organization with a base in the mid-Atlantic states and New England" and about a decade ago was brought, in part, to Philadelphia.
"We do not know where the art is currently located," he said, describing the paintings' fate as having been "secreted, unseen and unappreciated."
Officials said they wanted to spread news about the unclaimed $5 million reward for information leading to the paintings' recovery and to appeal for tips.
Ortiz also put out another piece of bait: immunity from prosecution.
There is "potential for immunity to anyone connected," she said, adding that there was no guarantee of "blanket immunity without knowing the specifics."
The apparent progress in one of history's greatest art thefts, also including works by Manet and Renoir, comes exactly 23 years since the thieves conned their way into the museum after hours.
Once in, they tied up the two guards and "roamed the galleries," according to the account from the Isabella Stewart Gardner Museum. Only the next morning when the new shift arrived were the bound guards -- and the robbery -- discovered.
© 1994-2013 Agence France-Presse