My understanding is that there was one murder committed by a legally owned class III Firearm...by a police officer in the 1970s killing his wife.
There have been three, the one you describe, another on September 15th, 1988, when a 13-year veteran of the Dayton, Ohio police department, Patrolman Roger Waller, then 32, used his fully automatic MAC-11 .380 caliber submachine gun to kill a police informant, 52-year-old Lawrence Hileman. Patrolman Waller pleaded guilty in 1990, and he and an accomplice were sentenced to 18 years in prison; no charges relating to the use/possession of the weapon were filed. And a Piggly-Wiggly grocery store owner in the south who used a Thompson in the 1960s in a killing of a *Dixie Mafia* member, who was arrested on state charges of possession of an illegal automatic firearm but was released after he was shown to be an undercover informant for the FBI, and therefore a *Police Agent* entitled to possess the weapon under state law. The feds declined to press any federal charges and the state homicide charges were also dropped.