It occurred as part of the attempt by a gang led by Vernon C. "Verne" Miller to free Frank "Jelly" Nash, a federal prisoner.

At the time, Nash was in the custody of several law enforcement officers who were returning him to the U.S. Penitentiary at Leavenworth, Kansas, from which he had escaped three years earlier.

On March 28, 1918, Nash's sentence was reduced to ten years after he convinced the warden he wanted to join the army and fight in World War I.

On March 3, 1924, Nash began a 25-year sentence at the United States Penitentiary, Leavenworth, in Kansas, for assaulting a mail custodian.

[citation needed] The FBI launched an intensive search for Nash throughout the entire United States and most of Canada.

After a thorough investigation, the FBI concluded that Nash had assisted in the escape of seven prisoners from the United States Penitentiary, Leavenworth, on December 11, 1931.

The investigation also disclosed that Nash had a very close association with Francis L. Keating, Thomas Holden, and several other gunmen who had participated in several bank robberies throughout the Midwest.

The pair had crucial information about the whereabouts of Nash and eventually divulged that he was hiding out in Hot Springs, Arkansas.

The three officials then drove Nash to Fort Smith, Arkansas, to board a train bound for Kansas City, Missouri, at 8:30 that night.

Before traveling, the lawmen contacted Reed E. Vetterli, Special Agent in Charge (SAC) of the FBI's Kansas City office, to meet them at the train station upon arrival.

Vernon Miller was designated to free Nash, and while at Mulloy's tavern in Kansas City, he made several phone calls for assistance in the scheme.

According to the official FBI report, Charles "Pretty Boy" Floyd and his sidekick Adam Richetti arrived in Kansas City to aid in the mission.

According to the FBI report, Floyd and Richetti happened to be on the way to Kansas City but had been detained at Bolivar, Missouri, early on the morning of the 16th, when the car in which they were riding became disabled.

Early the next morning, according to the FBI account, Miller, Floyd, and Richetti drove to Union Station in a Chevrolet sedan.

Frank Nash was handcuffed throughout the trip from the train to the Chevrolet, which was parked directly in front of the east entrance of Union Station.

At this point, from a distance approximately 15 feet diagonally to the right of Agent Caffrey's Chevrolet, an individual crouched behind the radiator of another car opened fire.

Just then a Kansas City policeman emerged from Union Station and began firing in the direction of one of the killers, later identified as Floyd, who slumped briefly but continued to run.

The investigation developed evidence that the scheme was carried out by Vernon C. Miller, Adam C. Richetti, and Pretty Boy Floyd.

The evidence included latent fingerprint impressions located by FBI Agents on beer bottles in Miller's Kansas City home and identified as those of Adam Richetti, thus helping to link the latter to the crime.

[citation needed] On November 29, 1933, during the FBI's search for Miller, his mutilated naked body was found in a ditch on the outskirts of Detroit, Michigan.

After an intensive search, the FBI and a team of local police officers located Pretty Boy Floyd hiding on a farm just outside Clarkson, Ohio, on October 22, 1934.

The four individuals who aided in the conspiracy—Richard Galatas, Herbert Farmer, "Doc" Louis Stacci, and Frank Mulloy—were indicted by a federal grand jury at Kansas City, Missouri, on October 24, 1934.

On the following day, each was sentenced to serve two years in a Federal Penitentiary and pay a fine of $10,000, the maximum penalty allowed by law.

[8][9] On August 3, 2015, Gangland Wire documentary filmmaker Gary Jenkins published a 3-episode true crime story podcast about the Union Station Massacre.