Supposedly, there's a second incident, but details are lacking.
http://www.guncite.com/gun_control_gcfullau.html
Crime with Legally Owned Machine Guns
In 1995 there were over 240,000 machine guns registered with the BATF. (Zawitz, Marianne,Bureau of Justice Statistics, Guns Used in Crime [PDF].) About half are owned by civilians and the other half by police departments and other governmental agencies (Gary Kleck, Targeting Guns: Firearms and Their Control, Walter de Gruyter, Inc., New York, 1997.)
Since 1934, there appear to have been at least two homicides committed with legally owned automatic weapons. One was a murder committed by a law enforcement officer (as opposed to a civilian). On September 15th, 1988, 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. The 1986 'ban' on sales of new machine guns does not apply to purchases by law enforcement or government agencies.
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Thanks to the staff of the Columbus, Ohio Public Library for the details of the Waller case.
Source: talk.politics.guns FAQ, part 2.
The other homicide, possibly involving a legally owned machine gun, occurred on September 14, 1992, also in Ohio
http://members.cox.net/arporro/photos/Shooting.pdf