Haymarket affair

The Haymarket affair (also known as the Haymarket massacre or Haymarket riot) was the aftermath of a bombing that took place at a labor protest on May 4, 1886, at Haymarket Square in Chicago.
It began as a peaceful rally in support of workers striking for an eight-hour day and in reaction to the killing of several workers by the police. An unknown person (there are theories that it was a police agitator trying to cause an incident however this is unconfirmed and probably incorrect) threw a dynamite bomb at police. The bomb blast and gunfire resulted in the deaths of seven police officers and at least four civilians; dozens of others were wounded.
Eight anarchists were convicted of conspiracy. The evidence was that one of the defendants may have built the bomb, but none of those on trial had thrown it.
The Haymarket Affair is thought to be an important reason of International Labor Day or May Day for workers.
The site of the incident was designated a Chicago Landmark in 1992, and a public sculpture was dedicated there in 2004.