On February 14, 1929, five members of the North Side Gang, plus gang collaborators Reinhardt H. Schwimmer and John May, were lined up against the rear inside wall of the garage at 2122 North Clark Street, in the Lincoln Park neighborhood of Chicago's North Side, and executed. Two of the shooters were dressed as uniformed police officers, while the others wore suits, ties, overcoats and hats, according to witnesses who saw the "police" leading the other men at gunpoint out of the garage after the shooting. John May's German Shepherd, Highball, who was leashed to a truck, began howling and barking, attracting the attention of two women who operated boarding houses across the street.
One of the women, Mrs. Landesman, sensed something was wrong and sent one of her roomers to the garage to see what was upsetting the dog. The man ran out, sickened at the sight. Frank Gusenberg was still alive after the killers left the scene and was rushed to the hospital shortly after real police officers arrived at the scene. When the doctors had Gusenberg stabilized, police tried to question him but when asked who shot him, he replied, "I'm not talking," despite having sustained fourteen bullet wounds.
George "Bugs" Moran was the boss of the long-established North Side Gang, formerly headed by Dion O'Banion who was murdered by four gunmen five years earlier in his flower shop on North State Street. Everyone who had taken command of the North Siders since O'Banion's rule had been murdered, supposedly by various members or associates of the Capone organization. This massacre was allegedly planned by the Capone mob in retaliation for an unsuccessful attempt by Frank Gusenberg and his brother Peter to murder Jack McGurn earlier in the year and for the North Side Gang's complicity in the murders of Pasqualino "Patsy" Lolordo and Antonio "The Scourge" Lombardo – both had been presidents of the Unione Siciliana, the local Mafia, and close associates of Capone. Bugs Moran's muscling in on a Capone-run dog track in the Chicago suburbs, his takeover of several Capone-owned saloons that he insisted were in his territory, and the general rivalry between Moran and Capone for complete control of the lucrative Chicago bootlegging business were probable contributing factors to this incident.
The plan was to lure Bugs Moran to the SMC Cartage warehouse on North Clark Street. Contrary to common belief, this plan did not intend to eliminate the entire North Side gang – just Moran, and perhaps two or three of his lieutenants. It is usually assumed that they were lured to the garage with the promise of a stolen, cut-rate shipment of whiskey, supplied by Detroit's Purple Gang, also associates of Capone. However, some recent studies dispute this, although there seems to have been hardly any other good reason for so many of the North Siders to be there. One of these theories states that all of the victims (with the exception of John May) were dressed in their best clothes, which would not have been suitable for unloading a large shipment of whiskey crates and driving it away – even though this is how they, and other gangsters, were usually dressed at the time. The Gusenberg brothers were also supposed to drive two empty trucks to Detroit that day to pick up two loads of stolen Canadian whiskey.
On St. Valentine's Day, most of the Moran gang had already arrived at the warehouse by approximately 10:30 AM. Moran was not there, having left his Parkway Hotel apartment late. As Moran and one of his men, Ted Newberry, approached the rear of the warehouse from a side