The Saint Valentine's Day Massacre was the murder of seven members and associates of Chicago's North Side Gang that occurred on Saint Valentine's Day 1929. The men were gathered at a Lincoln Park, Chicago garage on the morning of February 14, 1929. They were lined up against a wall and shot by four unknown assailants, two of whom were disguised as police officers.

The murders resulted from the competition for control of organized crime in the city during Prohibition between the largely Irish North Siders, headed by George "Bugs" Moran, and their largely Italian Chicago Outfit rivals led by Al Capone.^[1] The perpetrators have never been conclusively identified, but former members of the Egan's Rats gang working for Capone are suspected of involvement; others have said that members of the Chicago Police Department who allegedly wanted revenge for the killing of a police officer's son played a part.

https://en.wikipedia.org/wiki/Saint_Valentine%27s_Day_Massacre