During the Forty-First Congress, the Columbia Organic
Act of 1871 openly states the fact:

Be it enacted by the Senate and House of
Representatives of the United States of America in Congress assembled, That all
that part of the territory of the United States included within the limits of
the District of Columbia be, and the same is hereby, created into a government
by the name of the District of Columbia, by which name it is hereby constituted
a body corporate for municipal purposes, and may contract and be contracted
with, sue and be sued, plead and be impleaded, have a seal, and execute all other
powers of a municipal corporation not inconsistent with the Constitution and
laws of the United States and the provisions of this act.