gang boss Al Capone ambushed rival George Moran in the infamous St. Valentine's Day Massacre. The gang members were disguised as police. The real police arrived later and found shell casings and bullets on the victims. A ballistics expert, saw the shell casings were from Thompson sub-machine guns. He tested them against police guns. They didn't match. When police raided the home of Capone's hit men and recovered murder weapons, it was confirmed they were responsible.