BOSTON -- An inmate at MCI Norfolk said he knows the whereabouts of about $500 million in art stolen almost 20 years ago from a Boston museum.

The March 18, 1990, Isabella Stewart Gardner Museum heist is the largest art theft in modern history. Inside the museum, empty spaces where the 13 missing masterpieces once hung are constant reminders that the crime remains unsolved.

Robert Beauchamp said one of the suspects in the case, George Reissfelder, told him that the pieces were stashed in the walls of a "safe house" in Maine, Beauchamp's attorney, Alec Sohmer, said.

Beauchamp claims Reissfelder and two other people stole the art with the plan of negotiating its return for a lesser sentence for an unrelated crime and stored it in a Maine home, Sohmer said. The homeowner and Reissfelder both died before the art could be retrieved, Sohmer said.

Beauchamp has been working with the FBI and the museum to locate the safe house, Sohmer said.

The U.S. attorney said in 2005 that they may be willing to grant immunity from prosecution to anyone returning the masterpieces. The thieves themselves can never be prosecuted because the statute of limitations on their crime ran out years ago.

As a result of the Gardner heist, the statute of limitations on museum theft was extended in this country from five years to 20 years.