The location is key for a plot to work. Roads, buildings, fences, and other structures must be situated such that movements of the assassin are concealed while allowing the assassin to be in close enough range to get off a kill shot. Unless the assassin is suicidal, the assassin will want a location and set up that gives him/her a reasonable chance of escaping without being shot or captured.
A key question then is — Who specifically selected the fairground at Butler for the site of the Pennsylvania rally? Examine the players involved in the selection of the site, and placement of the stage, and you will have a path for discovering other conspirators and participants.
My guess is that they would have to have multiple people in place for different rallies - cannot rely on one plan going perfectly and relying on one person to go through with it