I read somewhere else that he had just started to work there, so he probably didn’t know anyone very well. And certainly not the higher ups.
In any case, he had to steal an ID to get in, so while he may have worked for them in some capacity, it doesn’t sound as if he was permitted free range of the yard.
Ah, if he just started working there, that goes to my earlier speculation that he had developed grudges against people in that bldg.
I would guess he used a stolen ID as misdirection.