Wikipedia: Saint Valentine's Day Massacre

History.com: St. Valentine's Day Massacre - Victims, Evidence & Suspects

Britannica: Saint Valentine's Day Massacre

Chicago Magazine: The St. Valentine's Day Massacre and Al Capone

Crime Museum: Saint Valentine's Day Massacre

The Mob Museum: St. Valentine's Day Massacre

Chicagology: 1929—St. Valentine's Day Massacre

Chicago Tribune: The St. Valentine's Day Massacre

Encyclopedia of Chicago: St. Valentine's Day Massacre

CBS News: Chicago Hauntings: Cursed Bricks, Noises, And Poltergeists Follow The Never-Prosecuted St. Valentine's Day Massacre

Forbes: Not So Sweet: The St. Valentine's Day Massacre

ThoughtCo: St. Valentines Day Massacre