The North Siders were assembled at the SMC Cartage Company garage, 2122 North Clark Street. A team of professional killers, two of them dressed as police officers, entered the building. Believing they were being raided by authorities, Moran's men cooperated and lined up facing a wall of the garage.
The helpless gangsters were then slaughtered in a hail of machine gun and shotgun fire. The killers escaped.
Gang boss Moran, said to be the primary target of the attack, also escaped. Lookouts working with the hit team mistakenly believed Moran was present in the garage and initiated the attack too early. According to reports, Moran was just approaching the building when he observed what looked to be a police raid and decided on a different course. When he learned of the massacre, he went into hiding.
The victims of the "St. Valentine's Day Massacre":
- James Clark, 42. The top lieutenant of George Moran (and often referred to in the press as Moran's brother-in-law), Clark (born Albert Kachellek) had been imprisoned several times for robberies and parole violation.
- Frank Gusenberg, 36. Often a suspect in burglaries and robberies, he served one jail sentence for disorderly conduct. He was considered an enforcer for Moran. He was the only one of the seven victims still living when police arrived. He died hours later.
- Peter Gusenberg, 40. The brother of Frank Gusenberg, he was the top enforcer of the Moran bootlegging operation. He served several prison terms for robberies and parole violation.
- Adam Heyer, 40. He had been in and out of prison since 1908, convicted of robberies, confidence games and parole violation. It was reported that Heyer managed the gang finances and ran the S.M.C. Cartage Company.
- John May, 35. A former thief, he was an associate of the Moran gang and worked as a automotive mechanic.
- Albert Weinshank, 35. A member of the Chicago cleaning and dying association, authorities believed he joined the gang when Moran was scheming to take control of that industry.
- Reinhardt Schwimmer, 30. An optometrist, he often socialized with the Moran gang and bragged of his underworld association.