Local Nationals are a common fixture at every U.S. Army site, perform many functions. Even at the unit level you can hire details of local nationals to do police calls, fill sand bags, we had Iraqi Contractors working on Generators, air conditioners, carpentry work. We assigned people in the unit to guard them while they worked and we were responsible to pay them ourselves, they always made me a little nervous. I personally did not find them particularly productive. We were encouraged to do this, winning the Hearts and Mind thing I guess.
Who did the screening?
Basically, an American presence anywhere on the globe.
Unfortunately, there's no way you're going to be able to resolve it aside from completely walling yourself off from the population.
I just hope and pray that future attacks like this can be preempted before they come to fruition.