Twenty-three years since 13 priceless pieces of art were stolen from the Isabella Stewart Gardner Museum, the Gardner and the FBI issued a public appeal similar to the one launched just prior to James "Whitey" Bulger's capture — complete with wanted posters.

The posters don't sport the usual most-wanted suspects. instead, they display the missing artwork in an effort to get someone to come forward with what they know about the most significant art heist in history.

Gardner Museum director Anne Hawley is opening up about the loss.

"It was such a painful and horrible moment in the museum's life," Hawley said.

Until now, Hawley has said little about the theft and what happened in the immediate aftermath.

"We also are being threatened from the outside by criminals who want attention from the FBI, and so they were threatening us, and threatening putting bombs in the museum," she said. "We were evacuating museum, the staff members were under threat, no one really knew what kind of a conundrum we were in."

Meanwhile, the FBI followed thousands of leads worldwide, including in Ireland and Japan, and they believe they know who might have taken the art.

WGBH News' Emily Rooney interviewed Jeff Kelley, a special agent in the FBI's Boston field office, and a member of the art crime unit.

Emily Rooney: You have been in this for at least ten years.

Jeff Kelley: It is actually 11 years now I have been the investigator on this case.

Rooney: You essentially know who did it.

Kelley: Yes.

Rooney: Why can't you say?

Kelley: We have to temper what we put out there in the public, and we certainly want to get the assistance of the public and we feel it is important to kind of lay our cards out on the table and say we know who did it, and we know who is involved, but we need your help.

We still have an investigation here, and we still have to preserve the integrity of the investigation, and because of that we can't tell you everything, and I know it is kind of a little tantalizing to kind of put that out there and not be able to follow it up and say this is who we think did it.

Rooney: Have you talked to the person?

Kelley: We have certainly talked a to a lot of people, we have spoken to people we think were involved and spoken to other people it has gotten us to where we are now, and basically we need the help of the public.

We have used it before, and it is great, and we continue to try and solicit the help from the public.

Rooney: Anne Hawley told us — we never heard that before — that right after the heist there were all kinds of bomb threats and the museum was threatened. Explain that. What happened?

Kelley: Certainly when you have a case of this magnitude, people are going to come out of the woodwork. That is what happened.

Shortly after the case — it happened over the years — where people came forward either claiming to have information about the theft, or coming forward to try and extort some money out of the museum, so this has been such an unusual investigation.

I have been working it for 11 years, but obviously it has been almost 24 years since it happened.

And it has run the gamut from everything from an art investigation to drug investigation to extortion investigation. I mean, it has really encompassed every type of federal statute that you could think of.

Rooney: The former Boston Herald reporter Tom Mashburger has a great tale of being, essentially, blindfolded and taken to a place where somebody unrolled something and he got some chips

Is there any possibility what he saw is one of the real pieces?

Kelley: I know Tom and he has the utmost integrity. But from what I have learned about the art itself, I don't think that what he saw was the actual painting.

He described it as being unrolled, kind of unfurled, but from speaking to the experts at the museum and at other museums, the paintings are so thick that they would really be almost impossible to roll up.

Rooney: Do you think that they are still in existence and do you think together — because with 13 objects, some of them are odd objects, they weren't all paintings — to think that they're together?

Kelley: I don't know if they are still together. I think they are all in existence.

You have to be cynical in this position and certainly one of the things we have to look at was: Did these paintings get destroyed right after? When these guys woke up and realized they committed the heist of the century, did they panic and destroy them? And that's why we haven't seen them — it is a possibility, but we have had confirmed sightings of some of these pieces throughout the 90s and into the early 2000s and that really gives us a comfort level that these paintings had not been destroyed.

Watch the Greater Boston segment: