My apartments opened my mail!?

I was checking my mail, which is sorted in to different boxes by people that work at the apartments. I found an open letter, the contents of the letter were not stolen, but it being a check may have detoured the would-be-thief. Who can i go to other than the apartments? I think this is a federal offense and I know the apartments are not going to do anything about it. I need some legal help i think.