36 C.F.R. § 520.2
Except as otherwise ordered, National Zoological Park buildings and grounds shall be closed to the public after posted visiting hours. Such buildings and grounds, or portions thereof, shall be also closed to the public in emergency situations and at such other times as may be necessary for the orderly conduct of business. Whenever the buildings and grounds or portions thereof are closed to the public for any reason, visitors will immediately leave the premises upon being requested by a police officer or other authorized individual. Admission to such premises during periods when closed to the public will be limited to authorized individuals who will be required to register and identify themselves when requested by police officers or other authorized individuals.
36 C.F.R. § 520.2