Washington, D.C. is the capital of the United States of America.
Description[]
As the U.S.A.'s capital, Washington D.C. was, by 1939, home to the American police's centralised bank of fingerprints records. During his investigation into the identity of the Phantom Blot, who had only recently begun terrorising Mouseton, Detective Casey sent Pinckley to Washington to check some fingerprints he'd found against the record. However, it turned out the prints the Detective had found belonged to Chief O'Hara himself and not to the Blot.
Behind the scenes[]
Washington, D.C., which of course exists in the real world, is mentioned in the 1939 story Mickey Mouse Outwits the Phantom Blot.