I think we have another prime suspect as part of the conspirator team:
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I think he was probably there to stage-manage what the police and other law enforcement think they know as soon as they arrive on the roof. And/or to confuse the situation. His primary tool for doing so was his phone, with which he pretended to talk to "other law enforcement" about what was happening on the roof and in the whole assassination attempt out loud. Notice that he also has an earpiece, through which he probably gets his information in real time. Here he is on the roof (starting at 3:51), right where the ladder is, and (all?) law enforcement people come up to the crime scene. He pretends to talk on the phone, telling details of "what happened" out loud which the police guys climbing up hear, while they think, by his manner of acting and what he says on the phone, that he is someone official from "another department", or something: