Mafia is best fit so far. Although you can still make a case for a rogue KGB operation (LHO's father-in-law was a KGB colonel, and some people think Marina was actually LHO's case officer [thus the book
Legend, 20 years ago]), ordered by Nikita Khruschev, who was ousted in 1964, possibly for ordering the operation? Just thoughts, so far.
But the Cosa Nostra story seems to fit pretty well with other known associations and coincidences.