This Is a Robbery goes along with this theory throughout episodes three and four. In an attempt to get his friend Vincent Ferrara out of jail with a bargaining chip, Mafia member Bobby Donati apparently set up a major heist to exchange stolen valuables for Ferrara's freedom. Someone even reportedly saw Donati sneak two police officer uniforms in a paper bag. If this was the case (and the FBI seems most convinced by this theory), mobsters likely cased the museum for its weaknesses before pulling off the heist. Donati, however, was murdered soon after the robbery.
One of the suspects in the heist was George Reissfelder, who matched the description of one of the fake police officers at the robbery. Reissfelder's former sister-in-law even recalled helping him hang a Manet piece (which, naturally, is nowhere to be found). When Donati went missing, his son contacted mobster Bobby Guarente, who had been connected to a crew that involved Reissfelder. According to his widow, Guarente gave some Gardner pieces to a man named Robert Gentile. As fate would have it, the FBI did not find any art at Guarante's. The main problem with the Mafia theory is that many of the allegedly involved members either died of natural causes or were murdered.