D.C. Code § 9-1201.09
The Secretary of the Interior is directed to prevent the improper appropriation or occupation of any of the public streets, avenues, squares, or reservations in the City of Washington, belonging to the United States, and to reclaim the same if unlawfully appropriated; and particularly to prevent the erection of any permanent building upon any property reserved to or for the use of the United States, unless plainly authorized by act of Congress, and to report to Congress at the commencement of each session his proceedings in the premises, together with a full statement of all such property, and how, and by what authority, the same is occupied or claimed. Nothing herein contained shall be construed to interfere with the temporary and proper occupation of any portion of such property, by lawful authority, for the legitimate purposes of the United States.
D.C. Code § 9-1201.09