Whiskey Tango Foxtrot.
This is BS, incidentally.
If the building was secured prior to the rally, all vehicle access roads would also be secured.
It's hard to drive down a secured road to a secured building, take out a ladder and a rifle and climb up the side of the building and take a dang near point blank shot at the former POTUS.
The preferred sniper hide is the back side of a sloped roof, every sniper learns to shoot from a roof in an urban environment.
Plus there's this thing called an observation platform, it has adjustable legs.