Officer David O'Brien and eight other members of the Milwaukee Police Department as well as a civilian were killed when a bomb exploded in the department's central police station. The bomb had been found near a church and was brought to the police station by two boys. As the station keeper was showing it to the shift commander, it exploded. Although the case was never solved, it is thought that the bomb was planted by anarchists.
Officer O'Brien had served with the agency for 20 years. He was survived by his second wife, two sons, and mother, and was predeceased by his first wife and infant daughter. He is buried in Calvary Cemetery and Mausoleum, Milwaukee, Wisconsin.