See: "Family Jewels" https://en.wikipedia.org/wiki/Family_Jewels_(Central_Intelligence_Agency) Hoffa was betrayed by a close friend, delivered to a mob asset, shot and then cremated, which is why his body will never be found. Given the players involved, the person who actually did the murder is almost certainly dead, so you are looking for whoever died around the same time as Hoffa. While many were involved, as named by Sheeran, Sheeran was also the puppet master (as he was working for the mob at the behest of military intelligence) and helped cover up the murder by giving wrong details about the case and leading investigators awry.