See:
http://www.dhs.gov/topic/federal-building-security
http://en.wikipedia.org/wiki/United_States_Federal_Protective_Service
My local SS office in Ft. Myers had a Glock-armed guard who kept us geezers from getting too rowdy so that we could hear our numbers called when we were up to see the staff.
I was surprised to see the Glock and I looked closely at his badge and it said “Federal Protective Service.”
This is NOT a “rent-a-cop” operation, IMO, and the officer I saw appeared to be extremely fit and likely former military, possibly a former Marine.
These officers appear to have 100% police powers on federal property and inside federal buildings.