Another angle to the JFK story to consider is that whoever the conspirators were, and whatever their motives were, Dallas was the place where they acted. One has to assume that the plot was not cobbled together on the morning of the visit. Thus the conspirators waited until Kennedy was in Dallas.
What follows from this? Either the conspirators were based in Dallas (such as Oswald), or they traveled to kill Kennedy, or a combination of the two. This would cast extra suspicion on people who knew or presumably had access to knowledge of Kennedy’s whereabouts. This hardly narrows it down, however. Various US government officials knew this, but from them informants could send this information outward to foreign governments, the Mafia, or any group sufficiently capable of maintaing some kind of intelligence service, broadly defined.
It also follows that the conspirators believed they could get away with the assassination in Dallas. (Maybe they could have gotten away with it anywhere, and Dallas just happened to be the spot where they did. Or maybe there was something special about Dallas.) In planning beforehand, they’d have to have a strategy for dealing with the authorities–either eluding the massive law enforcement presence or corrupting part of it.