They are reporting it on CNN. I'm watching the news so no, I do not have a link. They've said it multiple times now. That he worked for a Private Security Firm and has worked for the US Government (meaning he was sent out through his firm to work for them... probably as security for events/individuals).
It's basically impossible to stop a "lone wolf" type scenario (considering availability of guns in the US). As far as I can tell, this guy had no criminal record, was born in the US. So what do you do to stop someone like that?