D.C. Code § 5-202
The United States Park Police shall be under the exclusive charge and control of the Director of the National Park Service. It shall consist of an active officer of the United States Army, detailed by the Department of the Army, 1 lieutenant with grade corresponding to that of lieutenant (Metropolitan Police), 1 first sergeant, 5 sergeants with grade corresponding to that of sergeant (Metropolitan Police), and 54 privates, all of whom shall have served 3 years to be with grade corresponding to private, class 3 (Metropolitan Police); all of whom shall have served 1 year to be with grade corresponding to private, class 2 (Metropolitan Police) and such others as the Director of the National Park Service deems necessary and are appropriated for by Congress; and all of whom shall have served less than 1 year to be with grade corresponding to private, class 1 (Metropolitan Police).
D.C. Code § 5-202