Violence escalated on May 4 when a protest meeting began in Haymarket Square. During this meeting to denounce the events of the previous days, the police began to disperse the crowd when someone threw a bomb. Policeman Mathias J. Degan was killed almost instantly and seven other policemen later succumbed to injuries. Some of the speakers earlier in the day had been anarchists, and so the crime was presumed to have been committed by an anarchist, despite the fact that no evidence for such a link could be demonstrated.
Although nobody ever identified the bomb-thrower, eight men who were involved in organising the rallies were accused of the crime and found guilty. Seven of the men were sentenced to death and the eighth was sentenced to fifteen years in prison by Judge Joseph Gary, despite a startling lack of evidence that any of them had any role in the bombing. The sentencing sparked outrage in international labor circles, resulting in protests all around the world.
Eventually, Albert Parsons, August Spies, George Engel, and Adolph Fischer were hanged. Louis Lingg committed suicide in his cell. On June 26, 1893, Illinois governor John Peter Altgeld signed a pardon for Samuel Fielden, Oscar Neebe, and Michael Schwab. Altgeld's predecessor Richard Ogelsby had already commuted Fielden and Schwab to life in prison after admitting their innocence. Altgeld's pardon not only freed the three remaining men, it also sealed his own political demise.
August Spies is widely quoted as having said at his execution: "The time will come when our silence will be more powerful than the voices you strangle today."
In 1889, a 9-foot tall bronze statue of a Chicago policeman was erected near the site of the riot. The statue was long a subject of debate and scorn. After being moved from its original location, it was blown up at least twice by the Weather Underground before being moved to the lobby of police headquarters.
The site is now marked by a bronze plaque about two feet square set into the sidewalk and reading:
- ''"A decade of strife between labor and industry culminated here in a confrontation that resulted in the tragic death of both workers and policemen. On May 4, 1886, spectators at a labor rally had gathered around the mouth of Crane's Alley. A contingent of police approaching on DesPlaines Street were met by a bomb thrown from just south of the alley. The resultant trial of eight activists gained worldwide attention for the labor movement, and initiated the tradition of "May Day" labor rallies in many cities."
- Designated on March 25, 1992
- Richard M. Daley, Mayor''