If you want to believe Sam Giancana’s memoir, he killed her just before midnight (please remember Marilyn couldn’t have died later than 10 pm) at the orders of the CIA. However, he cannot exactly be trusted. Firstly, he was the boss of the mafia. So, honesty isn’t top of the priority list.
Secondly, he claims he is the reason Marilyn got famous. Except he clearly didn’t do a good job as she was paid lousy amounts in comparison to her co-stars, still had to work her butt off and was treated badly by her studio.
Thirdly, there is absolutely no reason he would’ve done so. The CIA had no reason to have Marilyn murdered and neither did the Mafia.
He didn’t know Marilyn. Yes, he knew Frank Sinatra and visited Cal-Neva Lodge. And yes, Marilyn also knew Frank and visited Cal-Neva Lodge. Does this mean they knew each other? No. It simply means they had a mutual acquaintance.
There is no proof other than a few books that claim Marilyn and Giancana were present at Lake Tahoe at the same time (and that she was raped by the Mafia whilst intoxicated).
It should be remembered that when a book comes out decades later and claims things when there are no surviving witnesses but it’s an exciting story that will sell, it doesn’t mean it’s going to be true.
The real question should be why would the Mafia wish to kill Marilyn? Why would the Kennedy’s have hired the Mafia? Especially when RFK had led some very tough battles against the mafia in the first place. In fact, JFK had to distance himself from Sinatra for his links to the Mafia… so why would they now risk everything and by employing them to kill a one-night stand?!