On February 14, 1929, seven men were murdered in a garage on Chicago's North Side in the St. Valentine's Day Massacre. The victims include five members of the North Side Gang (the Irish mob in Chicago) led by George "Bugs" Moran and two associates, and to associates of the gang. The men were murdered most likely by and/or on the orders of the Chicago Outfit (the Italian South Side gang and rival of the North Side Gang) led by Al Capone. While the exact identities of the massacre's perpetrators have never been discovered, it is believed that massacre was an attempt by Capone to eliminate Moran and possibly a few of his lieutenants. The promise of cheap, stolen whiskey from Detroit's Purple Gang was to lead Moran and his men to the garage to acquire a couple of trucks to retrieve the whiskey; however, Moran had left late and decided not to approach the garage upon seeing a police car parked nearby. Instead, one of Moran's men, who had dressed similarly to Moran and was of similar build, was mistaken for Moran by one of Capone's lookouts. This in turn set the massacre in motion as the lookout signaled for the ambush to start. Four men, two dressed of whom were dressed in police uniforms, pulled up in a car outside the garage. The "officers" entered through the front door and ordered the seven North Side men to line up against the wall; meanwhile, the other two men had entered through the back door and awaited their associates' signal. Then, the killers opened fire with Thompson submachine guns and shotguns, killing six and fatally wounding one of Moran's men. Afterwards, the men disguised as police led the other two out the front at gun point to give the illusion of being officers arresting the men and escaped in the car.

Only one man survived the ambush. Frank Gusenberg, one of Moran's enforcers, had survived 14 gunshot wounds and was taken to the hospital and stabilized. Despite questioning by Chicago police officers and detectives, Gusenberg refused to identify his killers. He died three hours later due to the severity of his wounds.