The National Postal Museum has an online exhibition, "Postal Inspectors: The Silent Service," which details the duties of postal inspectors in thwarting a wide variety of crimes, ranging from credit cards stolen from the mail and hazardous materials sent through the mail to an elaborate hoax that involved McGraw-Hill and Howard Hughes (both innocent parties). Go here for a timeline of Postal Inspection Service history, including the establishment of postal forensic laboratories in 1940. Who knew that the Postal Inspection Service was the first law enforcement agency to purchase the Tommy gun?
About the image: Postal inspector John Philip Crum on muleback during his journey through Alaska, ca. 1898. Library of Congress, Prints and Photographs Division