Hoffa was murdered by the Mob (Giacalone family) in Detroit and his remains are probably part of I-75 in Detroit, that's it.
He knew too much, had too much dirt on everybody else (including J. Edgar Hoover, who died in ‘72) and interestingly enough was eliminated (in 1975) before the HSCA could call him as a witness as they conducted a reinvestigation into the JFK assassination in 1976.
He was killed not just for the JFK/RFK connection but for many other reasons as well.
I am not making the linkage to the JFK case just in order to invent yet another, deeper conspiracy theory.
The Mob was involved in many “hits” including JFK's. Hoffa and the Kennedy brothers did not get along especially when RFK (as Attorney General) was putting intense heat on the Teamsters, Mob, AFL-CIO, UAW and the Teamster Truckers in the early 60’s.
Hoffa hated them and knew who killed them both, therefore, he had to go too.
Large concrete slab found at dig site per Drudge