More than two decades after the largest art theft in U.S. history, the FBI has renewed the offer of a reward for help in recovering the art and says it knows who is responsible for the theft.

The FBI as well as Boston's Isabella Stewart Gardner Museum and the Massachusetts U.S. Attorney's Office made the announcement Monday, offering $5 million to anyone who can help find the 13 pieces of art -- including rare paintings by Rembrandt and Vermeer.

"On the 23rd anniversary of the theft we are pleased to announce the FBI has made significant investigative progress," said FBI Special Agent Richard DesLauriers at a news conference.

DesLauriers wouldn't name the individuals the FBI believes is responsible, but said investigators believe they know who stole the art worth an estimated $500 million.

He said the thieves are members of a criminal organization based in the mid-Atlantic states and New England, and it is believed the art was transported over the years to the Connecticut and Philadelphia regions. However, investigators have no idea where the art is now.

Special Agent Geoff Kelly said he believes someone out there has seen the art and has the information the FBI needs to recover it.

"With these considerable developments in the investigation over the last couple of years, it's likely over time someone has seen the art hanging on a wall, placed above a mantle, or stored in an attic. We want that person to call the FBI."

The museum's head of security, Anthony Amore, explained that the reward is for information "that leads directly to the recovery of all of our items in good condition."

That means, he said, a simple phone call could be enough to earn the $5 million reward -- which can be paid anonymously through a lawyer.

The brazen heist took place in the early morning hours of March 18, 1990 -- one of the busiest days of the year for Boston police dealing with St. Patrick's Day revellers.

Two thieves disguised as police officers talked museum guards into letting them in a side entrance. Once inside, they tied up the guards and began methodically going through the museum, cutting key works of art from their frames, including pieces by Degas, Rembrandt, Manet and Vermeer.

They left with 13 pieces. Since then, the original frames have remained on the museum walls, sitting empty as a reminder of the missing paintings the museum hopes to one day repatriate.