On December 20, 1803, William C.C. Claiborne, Governor of the Mississippi Territory and one of the commissioners appointed to take possession of Louisiana from France, participated in the ceremonial exchange of the territory to the United States. Gov. Claiborne wrote to Secretary of State James Madison to announce the official transfer and notify Sec. Madison that a U.S. flag was raised over the city.
Letter to James Madison, Secretary of State, Announcing the Surrender of Louisiana to the United States by France, 12/20/1803, Records of the U.S. House of Representatives (ARC 306704)