Are you stupid
They rolled up in trucks in front, of course security is going to react that's why there was a firefight in the first place.
EDIT: Instead of insulting your intellience let me just explain to you my logic: Because Swat's (3) is based on America, I assume that the building the president is based in is just a mock-white house. The whitehouse has a laege courtyard in fornt of it, and beyond that is Penn. Ave, where te trucks roll up, at which point security reports this and attempts to inspect the trucks, however, Aribissian warriors jump out of the back and squeeze off the first round, and a firefight ensues.