according to their website, serving FCPD (and FCSO, towns of Reston and Herndon's PDs, and the FC Fire Marshall's office) to "provide the residents of Fairfax County with the most highly trained, competent, capable and professional law enforcement officers."
http://www.fairfaxcounty.gov/sheriff/academy.htm
I've been by there before when there's a lot of activity on the premises.
If SE corner, the Reconnaissance place.
Still going on, anyone?