The first full-time detective in Chicago was a Scottish-born immigrant named Allan Pinkerton. After he left the Chicago police force, Pinkerton founded the first detective agency in America, the Pinkerton Detective Agency, with the motto “We Never Sleep.” The symbol used for this agency was an unblinking eye, establishing the job title of "private eyes" for independent detectives.¹ The Pinkerton Detective Agency solved a series of train robberies prior to the American Civil War, gaining the attention of then Illinois lawyer Abraham Lincoln.

In 1861, while investigating a railway case, Pinkerton and his men uncovered an apparent assassination plot. Conspirators intended to kill Lincoln in Baltimore during a stop on his way to his inauguration.² Pinkerton warned Lincoln of the threat. The president-elect's itinerary was changed, giving him the advantage of secretly leaving the city at night.

After the Civil War ended, Allan Pinkerton returned to his detective agency in Chicago. His two sons, William and Robert, became his right-hand men. The company opened offices in New York City and Philadelphia. Bank and train robberies became frequent in the frontier west, and the Pinkertons were commissioned by railroad owners and government law officials to prevent further escalation of this crisis.³ When Allan passed away in 1884, his sons continued to lead the company with several thousand full-time employees and “reservists.”

REFERENCES
¹http://www.pbs.org/wgbh/americanexperience/features/biography/james-agency/
²http://www.encyclopedia.chicagohistory.org/pages/969.html
³http://privateinvestigatorforhire.com/private_investigator_history.html