1) The Government assassin inside the building took the first shots. They had the skill, but Trump moved, and the government assassin missed.
2) Hearing the first shots, Crooks — on the roof — swung into action, doing his part in the plan, firing into the crowd. He had probably been told that he’d be allowed to escape afterwards.
3) A government sniper on another roof, who had Crooks in his sights for quite a while, had waited patiently until he saw that Crooks had begun firing, and then took Crooks out, so that the “lone gunman” could be the fall guy for the operation.
There are cell phones (five right now that I know of, plus the Fox Mic) from many positions that picked up the shots. It should not be hard to triangulate on the position of every shot due to time differences of the shot sound.
This!