Criminologists label the people who kill several victims in a single short and bloody episode mass murderers. And, say the experts, these lethal criminals are increasingly common: Mass murder is not strictly a twentieth century phenomenon, but theorists believe that a breakdown in social controls during the past three or four decades has undermined the inhibitions that ordinarily keep a person from acting on the impulse to kill. Some authorities mark the dawn of the new age of mass murder as September 6, 1949, the day that Howard Unruh gunned down 13 people during a 12-minute walk through Camden, New Jersey. An individual's chances of being killed by a mass murderer are infinitesimally small. But the very fact that mass murders occur rouses in many the dreadful suspicion that life is a crapshoot: any one of us can run fatally afoul of a man who is consumed with rage and compelled to act it out.