Tough guidelines for the new facility -- the site must be within two miles of a Metro station, 2.5 miles from the Beltway and at least 55 acres -- have made sites in Fairfax County and Prince George's County the early favorites, lawmakers say.

Prince George's was thought to be in prime position after Metro and the General Services Administration, the federal government's real estate manager, agreed in December to group federal development near four Metro stations, two of which -- Branch Avenue and Naylor Road -- are in Prince George's.