If you watch the video of Jack Ruby shooting LHO as he walks past, you will notice that LHO is flanked by two cops. One of the cops is wearing a pale suit and pale cowboy hat. As he walks towards Ruby, the cop is flinching and looks ultra-nervous, like he is expecting Ruby will fire a gun. Dead giveaway.

Giancana on the surface had reason to want JFK dead, but I doubt he did it.

The Mafia might have been involved, or were maybe brought on-board by the government to help whack the President if the government didn't want the risk of using their own military/cop sharp-shooters. (Mafia acting alone probably wouldn't have gotten away with it without government backing. The government wouldn't cover up a mafia hit, unless the mafia were working in tandem with a government pissed at JFK.)