The Molly Maguires.

In the winter of 1876, testimony from a Pinkerton Detective Agency operative was used to destroy the Pennsylvania miners union, as 20 miners were accused of membership in a militant Irish organization called the Molly Maguires, convicted of murder, and hanged. The negative publicity from the trial effectively killed unionism in Pennsylvania mining for twenty years. This picture illustrated The Mollie Maguires and the Detectives, Allan Pinkerton’s self-serving account of his detective agency’s infiltration of the secret society of miners. Pinkerton’s work in the service of the Reading Railroad typified the widespread use of private police and organized violence by railroads and other businesses to suppress unions.

Source: Allan Pinkerton, The Mollie Maguires and the Detectives (1877)—American Social History Project.