A statement from this Boston.com newspaper article
“But in late April 1994, the museum received an overture that Gardner officials regard as the most promising lead ever in the case. An anonymous letter writer said he could facilitate the return of the paintings in exchange for $2.6 million and full immunity from prosecution for the thieves and those who held the paintings. Because the overture involved a request for immunity from prosecution, the museum turned the letter, postmarked in New York, over to the FBI.
The letter writer showed considerable knowledge of the paintings and of the international art world. He said the stolen paintings were being stored in archival conditions, and had not yet been sold. But, he warned, the museum should agree swiftly to the exchange because the paintings were being held in a country where a buyer who did not know they had been stolen could claim legal ownership.”
See image below for a Mr. Koch from "International Art Trading".