In explaining why mass murder has not been more thoroughly investigated by researchers, the authors argue that mass murder is not a significant challenge to police, given the ease of apprehension of suspects (or deceased suspects) in these cases. Also, mass murders do not connect well with the public in terms of eliciting fear; the public views them as catastrophes rather than events that are unfolding and have some potential to involve them as victims. Further, since most mass killers are either shot by police or commit suicide, there is little available data on them. Finally, mass murder does not have the same sensationalistic appeal that serial murder and serial murderers hold for the public. Following this overview of why there is so little research on mass murders, the authors used data analysis to examine two commonly held assumptions about mass murderers: that victims of massacres are usually strangers to their killer and that the characteristics of mass murders are such that they should be viewed criminologically as distinct and separate from single-victim homicides. The authors reviewed data from the Supplementary Homicide reports of the FBI for the years 1976-89. Incidents in this data set were classified as mass murder only if there were four or more victims. The final data set consisted of 329 "massacres" that involved more than 400 offenders and approximately 1,500 victims. Some 40 percent of mass murderers committed their attacks against family members and are often used to cover up other crimes. Mass murderers are more likely to be white, older, and male than single-victim homicide perpetrators. Mass murderers do not choose their victims randomly, but show a discernable relationship between victim characteristics and offender characteristics. Revenge is identified as the overwhelming motive in this form of murder, followed by love, profit, and terror. The authors develop a table of six contributing factors for the development of a mass murderer. 7 tables, 6 notes, and 23 references