Who killed John F KEnnedy?

I believe that the Mafia killed John F. Kennedy because he double-crossed them by playing both ends against the middle. I just finished reading "The Immortals" by Michael Korda. It's an old book (copyright 1992) that's been sitting around my house forever, about the Kennedy brothers and Marilyn Monroe. It was actually a very fascinating book. Those Kennedys sure make Bill Clinton look like a saint, though!!!