Captain Schaack was the most visible figure on the Chicago police force in the days after the bombing. Although he was not present in the Haymarket, he made sure that he was in the public eye during the arrests of anarchists that followed. He requested and received a great deal of independence and leeway from Superintendent Ebersold in hunting down those responsible for the bombing. Schaack boastfully attributed the successful prosecution to Inspector Bonfield and himself, who in his opinion succeeded despite the incompetence of the central administration of the department.

Lest he not receive enough credit, Schaack wrote his own book on Haymarket (albeit with considerable assistance from professional journalists), which emphasized his own exploits, the cowardice of many of those he arrested in the face of his authority, and the failures of Superintendent Ebersold. See the entry "Captain Schaack Speaks" in the section "The Appeal Continues" in Act V.

Captain Schaack commanded the fifth precinct, whose responsibilities included the North Side (i.e., north of the main branch and east of the north branch of the Chicago River) up to Division Street. He was based in the East Chicago Avenue station. During the Haymarket investigation, however, officers working under him, some in plainclothes and undercover, ranged over the whole city.

Captain Schaack and Inspector John Bonfield were among those officers who felt that Superintendent Ebersold was "soft" on organized labor. Schaack evidently convinced several businessmen of this, for, in addition to pledging financial support to the families of the dead and wounded policemen, they furnished him directly with funds (later reported by one newspaper as ultimately totaling a few hundred thousand dollars over a few years) to help conduct his investigations. He called these contributors, who included members of the Citizens' Association delegation that conferred with Mayor Harrison the day after the bombing, "public-spirited citizens who wished the law vindicated and order preserved in Chicago." Schaack claimed that between early May 1886 and November 20, 1887, nine days after the executions, he received 253 reports from his operatives. Their accomplishments are chronicled in his book, which is full of colorful if sometimes preposterous—and invariably self-serving—anecdotes.

Although he harshly criticized the anarchists as "exotics" who were foreign to the true America, Schaack was himself born in Luxembourg in 1843. When he was ten, his family emigrated to America, settling on a farm in Wisconsin after a brief stay in Chicago. At fifteen he was employed in a brewery in downstate Cairo, Illinois. He soon moved to Chicago, where he began his law enforcement career working for a private detective agency before joining the city police department in 1869. Schaack was promoted to captain in 1885, having established an enviable reputation for determination and ingenuity—if not for his attentiveness to proper legal procedure—based on the number and difficulty of cases he solved. A few days after he became captain he solved the notorious crime for which a man named Frank Mulkowski was hanged for bludgeoning to death his lover's stepdaughter.

In the third volume of History of Chicago, published the same year as Haymarket, Alfred T. Andreas attributed 865 criminal arrests in an eleven-year period to Schaack. "Among these were the most dangerous malefactors known in the West," Andreas added. Haymarket defense attorney Sigmund Zeisler, on the other hand, described him as "corpulent, pompous, inordinately vain and not overscrupulous." Somewhat more euphemistically, reporter Charles Edward Russell called Schaack "a man of restless and unregulated energy and, let us say, of small discretion."