A question about Area 51 and the use of deadly force by security officers?

I have heard that the use of deadly force is authorised on trespassers at Area 51 in America, now I know why such measure are only taken as a last resort to defend American security but is lawful to use deadly force on an unarmed civilian, I mean has a law ever been passed to allow security officers to shoot unarmed civilians and if so could anyone actually tell me what that particular law says