This certainly won't be believed by most but it is strongly suspected it was over the Alien file that JFK was going to expose. The head of CIA counterintelligence at the time, a fellow named Angleton if I'm recalling correctly, had written a memo that has been recovered and is in the possession of Dr Steven Greer (UFO guy) which lays out the pillow talk between JFK and Marilyn Monroe, killed for the same reason. Apparently she was threatening to call a press conference over it. JFK wanted to share info with the Russians and of course the cabal at the time was reaping the rewards of the reverse engineering they were doing on the Roswell and other recovered craft. So the CIA is really working on behalf of these Cabal who own the aerospace companies (they control everything because they control the printing press and merely gain control through intermediaries like Black Rock & Vanguard). Anyway, the hit was put on JFK/Monroe because of the threat of confirmation/exposure of the UFO phenomenon.