Not Mafia related but have you heard about the Union Station massacre in KC? In the '30s, gangsters machinegunned federal agents on the front steps of Union Station while they were transporting a prisoner killing him in the process. Supposedly bullet marks are still visible. Or how about when gangsters tommygunned the Benton County Sheriff while breaking one of their people out of the jail at Warsaw? Or how about the one armed Sheriff from either OK. or TX. who was after robbers that had killed people in his local and caught up with them swimming in Flat Creek at the old Grand Avenue ford creek crossing and shotgunned them all there in the creek?
Going back farther in time there's the Jesse James train robbery outside of Otterville. Quantrill hanging someone at Longwood during Sterling Price's raid.