The bullets were all from the same rifle.
Unlikely anyone was on the tower itself or the landing as it would be impossible to set up.
How do "they" (and we) know? The FBI has the recovered forensic evidence and has apparently not given out any information.
Did the eight recovered shell casings match the rifle extractor/firing pin signatures? Were they all the same and were they all from the same rooftop recovered weapon? Were any test shots taken by the rooftop rifle afterwards to determine if cartridge/bullet marking characteristics were similar to any recovered fired bullets? Of the three people shot were any of these expended bullets recovered for evidence and forensic examination? Were any other expended bullets recovered from the scene for examination? Does everything match up? The FBI should have all that evidence unless they carelessly discarded it along with the body they quickly identified as "Crooks" (oops!).
And by the way, is the recovered body actually of Crooks? Does DNA and fingerprint examination match? Surely there was residual evidence in his home of this, at least, and it was searched that night.
Questions, questions,,,?