In the United States and Europe, the distinction between public mass shooters and suicide terrorists no longer seems particularly meaningful. A number of public mass shooters have considered using bombs and claimed to be sacrificing themselves for an ideological cause, and many suicide terrorists have attacked without organizational support, using firearms, for what appear to be largely personal reasons. Previous research has also documented several common factors in these offenders' lives, including (a) suicidal motives and life indifference, (b) perceived victimization, and (c) desires for attention or fame. These factors are not always easy for observers to recognize in advance, so mental health professionals, the public, and law enforcement officials might need help from experts to more successfully identify at-risk individuals. This article reviews the evidence of each factor, provides a list of specific warning signs, and offers recommendations for future research. Ultimately, an evidence-based approach to prevention could help save both the lives of many potential victims and the lives of the would-be attackers themselves.