It's a small group, though - roughly 1000 people scattered over 148 locations. http://en.wikipedia....ne_Security_Guard and http://www.quantico....ID=2839&_r=search
Presumably they hire private contractors for additional "security" because of past decisions that it was cheaper and "more efficient" to have non-military and non-civil-service people doing those jobs. Penny wise and pound foolish, if you ask me...
Cheers,
Scott.