FWIW (and IIRC), it was widely circulated not long after the initial Snowden revelations that he was not an isolated case and there would be other revelations not linked to him. It would seem that an Angleton-like top-down review of vetting, across agencies, would not have been unreasonable at the time - but clearly wasn't done (or was, and the results were too embarassing).
I'm shocked.