By Alina Cho, CNN
(CNN) – It is the largest art heist in history.
For 20 years, investigators have been chasing down hundreds of leads. They've interviewed countless witnesses all over the world, and still the central questions remain: where is the art and who did it?
What happened on March 18th, 1990 at Boston's Isabella Stewart Gardner Museum? A a new portrait is now emerging about the famous heist, with some tantalizing details.
Investigators say at precisely 1:24 a.m., two men disguised as policemen knocked on the side door of the museum, saying they were called to look into a disturbance. The night watchman let them in.
Once inside, the thieves handcuffed both of the guards on duty, tied them up with duct tape and then, with free reign of the museum, they went to work.
What the pair took, didn't take, and how they did it, is as baffling to investigators as the crime itself.
"Certainly, they don't know a lot about art," says Geoffrey Kelly, special agent, FBI, Boston Division.
Among the 13 works stolen: three Rembrandts, including "The Storm on the Sea of Galilee," his only known seascape; five drawings by Degas; and "The Concert" by Vermeer, one of only 36 known Vermeers in the world.
But the thieves left hanging Titian's "Europa," the museum's most valuable piece estimated worth $400 million.
Then, there's the method they used: cutting some of the paintings out of their frames, which can damage them. Today, museum-goers see empty frames as a haunting reminder.
"That's the fear is that they rolled up two of the Rembrandts and that is a big fear because the paint will crackle," says Stephen Kurkjian, Boston Globe correspondent.
Kurkjian has been following the Gardner heist for the Globe.
"They missed a Michelangelo and they took something two steps away that had nowhere near the value," he says.
But the question remains, who is behind the biggest art heist in history? Over the years there have been wild theories. Was it a fugitive mob boss? An eccentric art collector? Or just the work of local criminals?
"There are so many good suspects, it's like an Agatha Christie novel where everybody's sitting in the living room and everyone has a particular motive as to why they committed the crime," says Kelly.
On the case for eight years, Kelly says DNA testing is now in play, but he won't reveal details.
The Boston Globe reports that investigators may be analyzing the duct tape used to silence the guards. If there's sweat on the tape, there's a possibility of a DNA match, and the break investigators have been hoping for all these years.
The FBI has taken out ads, placing billboards on the highway, offering a $5 million reward for any information that leads to the safe return of the artwork.
There are two crimes in the matter: the actual theft of the artwork, for which the statute of limitations ran out in 1995.
And then, there's the second crime: possession of stolen art. There is no statute of limitations on that, which is why the U.S. Attorney's Office is now offering immunity. Prosecutors say if someone comes forward with the art, all will be forgiven.