The FBI is described in Target: UCAS and mentioned in SoNA and a couple other places.
There are two FBI buildings in Washington, D.C. The first is the HQ--the Hoover building. The second is the Washington Field Office (WFO), which is down the street at 601 4th Street, N.W. (across the street (east) from the National Building Museum). Depending on what you want to do, they might
operate out of one or the other--as far as who's giving orders. Since WFO (aka Buzzard's Point) is where the FBI agents with jurisdiction in the immediate D.C. area work (the guys who do the every day stuff) and some of the CT/CI ops (one of the reasons why WFO is run by an Assistant Director of the FBI instead of a Special Agent in Charge) the major national security activities (CT/CI/Intel) are run out of the Hoover building, but are also represented in each Field Office (FO), with some (WFO, NYFO) being of notable distinction for the size and expertise of their national security/CT divisions.
The FBI maintains a section called the
Critical Incident Response Group. CIRG includes all of the special task units, including the Hostage Rescue Team, which is their special operations unit.
I assume that you want these guys to be covert, so they probably won't be working out in Quantico with FBI agents (oh, and the Marines next door). If it was all the same, they could be working through somewhere like WFO or through someone in CIRG, but not necessarily waiting in Quantico.