I would say that a compelling argument against the CIA being involved is that they would have done a way better job of covering their tracks if it really was them. Think about it: Professional spooks who know they would probably receive a death sentence for treason if caught planning an assassination attempt on the president.

And think about the number of people involved in an organisation like that who pledged allegiance to serve their country and now have to go along with killing the president. Just so incredibly unlikely. People watch too many cloak and dagger movies.