The guys who shot McGurn in a phone booth were the Gusenberg brothers, top gunmen in the Bugs Moran gang. The St. Valentine's Day Massacre was set up by McGurn as revenge for the shooting, not (as many people believe) by Capone to get rid of Moran. Capone fronted ~$10k to McGurn to carry out the massacre. McGurn checked into a hotel that day with his girlfriend (called "the blond alibi"). He constantly called to order room service, each time asking for the exact time of day. Capone was in Florida, meeting with the Dade County DA at the time. The massacre turned the tide against Capone. He grew disillusioned with McGurn, who was killed later in a bowling alley.