D.C. Code § 16-1381
In order to promote the orderly and proper development of the seat of government of the United States, agencies of the United States authorized by law to acquire real property, may acquire, in the public interest, by gift, dedication, exchange, purchase, or condemnation fee simple title to land (or rights in or on land or easements or restrictions therein) within the District of Columbia for public uses, works, and improvements authorized by Congress, in excess of that actually needed for and essential to their usefulness, in order to preserve the view, appearance, light, and air and to enhance their usefulness, to prevent the use of private property adjacent to them in such a manner as to impair the public benefit derived from the construction thereof, or to prevent inequities or hardships to the owners of adjacent private property by depriving them of the beneficial use of their property.
D.C. Code § 16-1381