Officials declined to release details ahead of a 2 p.m. ET press conference, where they plan to reveal "investigative developments" as well as a publicity campaign related to the theft.
The 13 paintings, including Rembrandt's "Storm on the Sea of Galilee" and Edouard Manet's "Chez Tortoni," were stolen on the night of March 18, 1990, when two men dressed as police officers arrived at the private museum's front door and a security guard let them in.
The thieves allegedly overpowered both guards, who were found duct-taped to chairs in the museum's basement the next morning.
The FBI solved Boston's other long-running crime mystery in June 2011, when it found accused mobster James "Whitey" Bulger hiding in a seaside California community. Bulger was arrested on a tip that came in after the FBI launched a publicity campaign aimed at tracking him down; he had been on the run since 1994.
The Gardner Museum was founded by Isabella Stewart Gardner, an art collector who died in 1924. Her will contained very particular conditions on the running of the museum, including the arrangement of her collection and free admission to anyone named Isabella, a practice it continues today.
(This version of the story corrects the spelling to "Stewart" from "Stuart" throughout.)
(Reporting by Scott Malone; Editing by Lisa Von Ahn)