In the U.S., the Pinkerton National Detective Agency was a private detective agency established in 1850 by Allan Pinkerton. Pinkerton had become famous when he foiled a plot to assassinate then President-Elect Abraham Lincoln. Pinkerton's agents performed services which ranged from undercover investigations and detection of crimes to plant protection and armed security. It is sometimes claimed, probably with exaggeration, that at the height of its existence the Pinkerton National Detective Agency employed more agents than the United States Army. 
Pinkerton agents were also hired to track western outlaws Jesse James, the Reno brothers, and the Wild Bunch, including Butch Cassidy and the Sundance Kid. The Pinkerton agency's logo, an eye embellished with the words "We Never Sleep," inspired the term "private eye."
It was not until the prosperity of the 1920s that the private investigator became a person accessible to the average American. With the wealth of the 1920s and the expanding of the middle class came the need of middle America for private investigators.