Police arrested hundreds of people, but never determined the identity of the bomb thrower. Amidst public clamor for revenge, however, eight anarchists, including prominent speakers and writers, were tried for murder. The partisan Judge Joseph E. Gary conducted the trial, and all 12 jurors acknowledged prejudice against the defendants. Lacking credible evidence that the defendants threw the bomb or organized the bomb throwing, prosecutors focused on their writings and speeches. The jury, instructed to adopt a conspiracy theory without legal precedent, convicted all eight. Seven were sentenced to death. The trial is now considered one of the worst miscarriages of justice in American history.
Many Americans were outraged at the verdicts, but legal appeals failed. Two death sentences were commuted, but on November 11, 1887, four defendants were hanged in the Cook County jail; one committed suicide. Hundreds of thousands turned out for the funeral procession of the five dead men. In 1893, Governor John Peter Altgeld granted the three imprisoned defendants absolute pardon, citing the lack of evidence against them and the unfairness of the trial.
Read more about the Haymarket incident at the Encyclopedia of Chicago. Link -via Metafilter, where you'll find many more links on May Day.