(Clever kid - bought a ladder that morning and knew right where to place it on the only building that had no protections, yet was 130 yards direct line of sight for a kill shot on a former President.)
Precisely! How would Crooks know that Secret Service agents weren’t on that roof?
Someone told Crooks where to go, to crawl along the roof and what angle he could shoot Trump from.
Someone also put that ladder there for him.
Maybe the rifle as well. He wasn't wearing concealment clothes and it would have been pretty obvious walking around with an AR.