On this date in 1917, a bomb exploded at the Milwaukee, WI Police headquarters. The bomb, a black powder device, was found outside a Catholic church. The church janitor brought it to the police station, where it exploded while being examined by officers. Nine police officers and a civilian employee were killed in the blast. The case was never solved, but an anarchist group was believed to be responsible.
City of Milwaukee – 1917 Bombing
Wikipedia – Milwaukee Police Department