IIRC one saw him up there scoping around, but thought it may be SS, as their plans weren’t well communicated to the lowest ranks, the cops on the ground. However, when other people described him one cop went up there. He started shooting immediately afterward. So he may have notified them ASAP, like on his radio, but it was too late. Crooks spot got blown and it forced him to act.
Or so this seems to be the narrative at this early point