All public buildings outside of the District of Columbia and outside of military reservations purchased or erected out of any appropriation under the control of the Administrator of General Services, and the sites of the public buildings, are under the exclusive jurisdiction and control, and in the custody of, the Administrator. The Administrator may take possession of the buildings and assign and reassign rooms in the buildings to federal officials, clerks, and employees that the Administrator believes should be furnished with offices or rooms in the buildings.