United States Postal Inspection Service founded by Benjamin Franklin
History
The Postal Inspection Service has the oldest origins of any federal law enforcement agency in the United States. It traces its roots back to 1772 when colonial Postmaster General Benjamin Franklin first appointed a "surveyor" to regulate and audit the mail. Therefore, the Service's origins predate the Declaration of Independence, and even the United States itself.
As Franklin was appointed Postmaster General under the Second Continental Congress, his system continued. One of Franklin's first acts as Postmaster General was to appoint William Goddard as the first Postal Surveyor of the newly founded American postal system, in charge of inspecting the integrity and security of postal routes, regulating post offices, and auditing their accounts. A letter from Franklin to Goddard, dated August 7, 1775, authorized a total of $170.00 for Goddard to carry out these duties, and August 7 is recognized as the "birthday" of the U.S. Postal Inspection Service.
In 1801, the title of "Surveyor" was changed to "Special Agent", and the Postal Inspection Service was the first federal law enforcement agency to use the title Special Agent for its officers. In 1830, the special agents were organized into the Office of Instructions and Mail Depredations. In 1873, one of their primary duties was the enforcement of obscenity prohibitions under the Comstock Act, named after Postal Inspector Anthony Comstock. Congress changed this title to "Post Office Inspector" in 1880 to differentiate the federal postal agents from the multitude of other "special agents" employed by railway and stagecoach companies. 
Finally, in 1954, the title changed again to "Postal Inspector" to reflect their relationship to all phases of the postal system and the U.S. Mail, instead of only post offices.