Top Secret/Extended Background Investigation clearance. Beyond that, it's not wise to describe it any further. But, you practically have to remember what you ate for breakfast when you were six. The clearance takes a year or more, and cost the government $40,000 back when I got mine. They go back and talk to all the people you associated with, including the people that may wanna shove a knife in your back. You have to retroactively mind all your p's and q's, and live a quiet life of an ascetic monk to make sure you don't get out of step with the security weenies, come "update" time (every 5 years). Unless, of course, you are a high-level Democrat.
So all the worker minions are read the riot act and monitored with electron microscopes, but high-level low-life Clintonistas seemed to waltz right past this intense scrutiny and got these clearances handed out like party flyers, and they had access to a helluva lot more than the minions did. It's like locking the back doors and windows with thirty bolt locks each, then flinging the front door wide open.
Speaking as a former Marine and knowing something about the security clearance process I was flabbergasted by the fact Clintonistas (starting with Bill & Hill) were able to get the most minimal security clearance, let alone top security. The Clinton gang certainly made a mockery of the system.