At 10.30 am on February 14 1929, seven men were murdered at the garage at 2122 North Clark Street, in the Lincoln Park neighborhood of Chicago's North Side.
They were shot by four men using weapons that included two Thompson sub-machine guns.
Two of the shooters were dressed as uniformed policemen.
Witnesses saw the ‘police’ leading the other men at gunpoint out of the garage after the shooting.
The victims included five members of George ‘Bugs’ Moran's North Side Gang.
When real Chicago police officers arrived at the scene, one of the victims, Frank Gusenberg was still alive.
He was taken to the hospital, where police tried to question him. Asked who shot him, Gusenberg, who had sustained fourteen bullet wounds, replied, "No one shot me." He died three hours later.
The massacre was allegedly planned by the organisation led by gangster Al Capone to eliminate Bugs Moran, the boss of the long-established North Side Gang.
Several factors contributed to the timing of the plan to kill Bugs Moran and some of his lieutenants.
On St. Valentine's Day, most of the Moran gang had already arrived at the warehouse but 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 they saw a police car and immediately turned and retraced their steps, going to a nearby coffee shop.
Al Capone's lookouts likely mistook one of Moran's men for Moran himself – probably Albert Weinshank, who was the same height and build.
Witnesses outside the garage saw a Cadillac sedan pull to a stop in front of the garage. Four men, two dressed in police uniform, emerged and walked inside.
The two fake police officers, carrying shotguns, entered the rear portion of the garage and found members of Moran's gang and two gang collaborators, Reinhart Schwimmer and John May, who was fixing one of the trucks.
The fake police officers ordered the men to line up against the wall and two of the killers opened fire with Thompson sub-machine guns, one with a 20-round box magazine and the other a 50-round drum.
They were thorough. The seven men were ripped apart in the volley.
Al Capone had an alibi – he was at home during the event, so was not arrested until years later on tax evasion charges.
Today, the surviving bricks from that garage massacre are on display at the Mob Museum in Las Vegas, which opened on February 14, 2012.