The Ariel Rios Federal Building is located in the Federal Triangle in Washington, D.C., across 12th Street from the Old Post Office, which the new building was designed to replace. The New Post Office, as the Rios Building was originally known, housed the headquarters of the Post Office Department until that department was replaced by the United States Postal Service in 1971. The building, which now houses the headquarters of the U.S. Environmental Protection Agency, was renamed on February 5, 1985, in honor of Ariel Rios, an undercover special agent for the Bureau of Alcohol, Tobacco, Firearms and Explosives who was killed in the line of duty on December 2, 1982.
The Rios Building was constructed in the early 1930s as part of the redevelopment of the Federal Triangle area. At that time one of the city’s most blighted neighborhoods, this area was known as "Murder Bay" and was a center of crime and prostitution. The organization for area's redevelopment was ordered out as part of the 1901 McMillan Plan, the first federally funded urban redevelopment plan, and the redevelopment of Federal Triangle began in earnest in the 1930s under the leadership of Treasury Secretary Andrew W. Mellon. Construction on the Rios Building was completed in 1934.