On the morning of June 17, 1933, a mass murder committed in front of Union Railway Station, Kansas City, Missouri, shocked the American public into a new consciousness of the serious crime problems in the nation. Four law enforcement officers were killed in an incident most often remembered as The Union Station Massacre, or also as The Kansas City Massacre.

The Kansas City Massacre occurred when the Pretty Boy Floyd gang attempted to free gang member Frank Nash. Nash was being transported from Oklahoma to the U.S. Penitentiary in Leavenworth, Kansas. Unknown to officers, an informant had relayed all the information regarding the arrival of Nash and his law enforcement escorts to various gangsters throughout the area. As the group of officers and agents entered their cars at Union Station in Kansas City, the gang opened fire. In the ensuing gun battle Detective Frank Hermanson and Detective William Grooms of the Kansas City Police Department, Chief of Police Otto Read of the McAlester Police Department in Oklahoma, and Special Agent Raymond Caffrey of the Federal Bureau of Investigation, were shot and killed. Frank Nash, the suspect the gang was trying to free, was also shot and killed.

Pretty Boy Floyd was shot and killed the following year during a shootout with officers. A second gang member was arrested and executed for the murders, and the third gang member was found dead from an unrelated murder.

Being a former KCPD officer, I've been able to examine much of the evidence gathered at the scene that day which is still in our crime lab’s inventory. And one can still go to Union Station and count bullet-made pock marks in the exterior facade of the building.

http://www.kchistory.org/week-kansas-city-history/mess-massacre