BOSTON — Although more than 100 miles away from where the masterpieces were stolen, FBI investigators believe that Portland plays a central role in solving the 25-year mystery of what remains the largest art heist in world history: the theft of artwork from the Isabella Stewart Gardner Museum.

Specifically, investigators are interested in what took place at a Portland seafood restaurant where Robert F. Guarente and Robert V. Gentile, two aging criminal associates who had long been friends, sat down with their wives in 2003 to enjoy lunch.

During their chit-chat, Guarente, who’d been freed from federal prison on a cocaine trafficking conviction the year before, told the Gentiles that he was dying of cancer and he was living his last days at his home in the central Maine town of Madison.

Years later, Gentile (who couldn’t recall the restaurant’s name) and Guarente’s widow agreed that she had ordered twin lobsters for lunch, but they had contradictory accounts on what had taken place after they had finished their meals. Elene Guarente told FBI agents in 2010 that the two men went into the restaurant’s parking lots and her husband opened the trunk of their car and handed over two or three paintings to Gentile.

Although she was fuzzy on some details, Elene Guarente said she believed the paintings were among the 13 pieces of art stolen from the Isabella Stewart Gardner Museum in 1990 and that they had long been held by her late husband at their home in Madison.

Although FBI agents have worked tirelessly chasing down hundreds of leads in the 25 years since the paintings were stolen, what Elene Guarente told them had taken place at the Portland restaurant remains the best tip investigator say they’ve received.

Elene Guarente, who still lives in Maine, was unsure why her husband had given Gentile the paintings – perhaps for safekeeping by Gentile or to turn over to another criminal associate. But she told the agents – and later a federal grand jury – that Gentile had placed the paintings in the trunk of his car and driven off with his wife back to their home outside Hartford.

Federal agents soon found reason to be heartened by what Elene Guarente had told them. When confronted with her account, Gentile acknowledged that he had in the past conversed with her husband about the Gardner theft and the two had schemed on how they might assist in getting back the stolen artwork and securing the $5 million reward that the museum offered for their recovery.

Later, the agents found, in the basement of Gentile’s home, a handwritten note on a sheet of typewriter paper that set out what each of the stolen pieces might bring on the black market.

Gentile insisted that it all amounted to empty talk between two old criminal associates and he never had gotten any paintings from Guarente after their lunch at the Portland restaurant. Nor did he know or believe that his old pal, Guarente, had the stolen Gardner paintings. The authorities, however, continue to believe Elene Guarente’s account and treat her version of the events at the Portland restaurant as the biggest break in the case.

But their efforts to gain a confession from Gentile have failed, though they charged him with involvement in criminal activity in 2012 and again this March. He is now on trial on charges he violated the terms of parole by meeting with other mobsters as well as selling a loaded pistol to a former convict.

Now 79, Gentile told a federal judge at a recent hearing that he believes he will die in prison because he is incapable of complying with federal prosecutors’ demands that he turn over the paintings stolen in 1990.