FBI: We know who carried out greatest art heist in American history

The FBI knows who pulled off the biggest art heist in history, but they aren't naming names. And as for what become of the $580 million worth of masterpieces stolen exactly 23 years ago from a Boston museum, investigators say that trail went cold a decade ago.

The FBI said Monday it would be "imprudent" to disclose the identities of the thieves who stole 13 works of art from Boston's Isabella Stewart Gardner Museum in 1990, but said they belong to a criminal organization, and they said they believe the art work has "changed hands several times" over the years.

"The FBI believes with a high degree of confidence in the years after the theft the art was transported to Connecticut and the Philadelphia region and some of the art was taken to Philadelphia where it was offered for sale by those responsible for the theft," Special Agent Richard DesLauriers said during a press conference.

"With that same confidence we have identified the thieves who are members of a criminal organization with a base in the mid-Atlantic states and New England," he said.

After the attempted sale in Philadelphia, which authorities claim took place about a decade ago, the investigators' knowledge of the art's whereabouts is limited.

Just after midnight on March 18, 1990, two men dressed as police officers buzzed the side door at the Boston museum and claimed they were there to investigate a disturbance.

A little more than an hour later, the men left with what is said to be the most valuable collection of stolen artwork in history: $580 million worth of famous works, including Rembrandt's only seascape, "The Storm on the Sea of Galilee," and Vermeer's "The Concert," a masterpiece valued at more than $200 million.

The FBI on Monday pleaded with the public for any information leading to the whereabouts of the famed works, which they said could be hanging over a mantel somewhere or hidden in an attic.

Investigators over the years have followed leads from Nevada to France, but the priceless items snatched from the museum have never been recovered.

The two men who broke into the museum -- hours after Boston celebrated St. Patrick's Day -- had "inside knowledge" of the museum's surveillance system, FBI Special Agent Geoff Kelly previously told FoxNews.com.

The suspects, described as white men in their 30s, were disguised as Boston police officers when they approached the museum door. The pair convinced two inexperienced security guards that they were responding to a call, before overtaking the guards and tying them up.

They spent 81 minutes inside the museum, walking the dark hallways before making their way to the Dutch Room, where the most valuable works were found.

The pair smashed glass and used box cutters to remove the masterpieces from their frames. In all, 13 priceless items were taken: three paintings by Rembrandt including, "The Storm on the Sea of Galilee," five drawings by Degas, and Vermeer's "The Concert" -- said to be the most valuable stolen painting in the world. The thieves also snatched an ancient Chinese bronze beaker or "Ku" from the Shang Dynasty and a finial that once stood atop a flag from Napoleon's Army.

But the method by which the pair seized the works led police to believe they were inexperienced art thieves.

"They were clever in how they got into the museum," Kelly said, "but the working profile points to inexperienced art thieves."

"How they went about removing the paintings – slicing them from their frames – that's indicative of a rank amateur when it comes to art theft," Kelly said. "Anyone who knows anything about art, when you’re taking an old Dutch master, slicing out of the frame will damage the painting."

The pair also made sure to cover their tracks. They took the museum's surveillance tape with them. They also took a printout from a computer that showed -- based on motion detectors -- where they had walked in the museum. That information, however, was already captured on the computer's hard drive, confirming to authorities where in the museum the thieves had been and how long they had stayed.

"They had a comfort level that really would establish they had some type of knowledge about how the security protocols were conducted at the museum," said Kelly.

Kelly said it's highly probable the thieves had no idea the magnitude of their crime until they woke up the next morning and realized they had committed the "heist of the century." He said it's possible they planned to "wait until the heat dies down" before attempting to sell the works. But it never did.

The museum has a $5 million reward for anyone with information on the whereabouts of the items. The U.S. Attorney in Boston is also offering immunity from prosecution for anyone with information leading to the stolen art.

Anyone with information on the Gardner heist is urged to call the FBI at 617-742-5533. Tips can also be submitted online at https://tips.fbi.gov.

For more information on the crime, visit www.FBI.gov/gardner.

FoxNews.com's Cristina Corbin contributed to this report.