When the Pinkerton Detective Agency was contracted by the state of Idaho to conduct an investigation into the Steunenberg assassination it sent to Boise the most famous detective in the land, James P. McParland. McParland had made his reputation thirty years earlier in the anthracite coalfields of Pennsylvania working undercover to expose a murderous gang of Irish-American thugs known as the Molly Maguires. Working for $12 a week, knowing he faced certain death if exposed, McParland frequented the card rooms and bars where Mollies were rumored to meet until we earned the trust of his targets and became a member of their secret society. When suspicions arose that McParland was an informer, he fled, dashing across frozen fields ahead of a gang of tomahawk-wielding toughs. McParland testified in nine Molly trials, helping to convict and execute twenty members, including most of the leadership, of the secret society responsible for so much Pennsylvania terror. McParland's Molly exploits earned him a cameo in Sir Arthur Conan Doyle's The Valley of Fear, where he and Sherlock Holmes have an encounter.

McParland specialty with the Pinkertons was labor unrest, making McParland, by then 62 and manager of Pinkerton's western operations, an obvious choice to head the Steunenberg investigation. Arriving in Boise in January of 1906, the portly master detective spent five hours going over details of the case with Idaho's Governor Gooding.  He announced his suspicion that Orchard, already captured, was "the tool of others," and requested that he be transferred from the jail in Caldwell to the state penitentiary in Boise so as to better extract a confession. The confession was soon in his hands after he suggested to Orchard that cooperation would likely lead to more lenient treatment. McParland then focused his attention on arranging the hasty arrests of the inner circle of the WFM implicated by Orchard in the Steunenberg assassination. McParland also arranged the special train that would carry the three inner circle members to Idaho. Those missions accomplished, McParland divided time between trying to round up potential witnesses, assembling incriminating evidence, leaking information that would tarnish the reputations of the defendants and their attorneys, checking out potential jurors, and orchestrating the prosecution effort. Although James Hawley announced in his opening statement for the prosecution that James McParland, "the terror of evildoers throughout the west," would be a witness, he was never called.

McParland was born in Ireland in 1843. He remained in Ireland and England for 26 years, working as a stock clerk, a fieldhand, a circus barker, and a chemical plant worker before taking a ship from Liverpool to New York in 1867. McParland settled in Chicago, where he opened a liquor store. When the Great Chicago Fire of 1871 destroyed his business, he took a job with the Pinkertons and began his colorful career as a detective.