["Behavioral Sciences &the Law, EarlyView. ", "\nABSTRACT\nThis is a retrospective case study of an antisemitic lone actor terrorist who completed the deadliest attack against the Jewish community in American history. The analysis through the lens of the Terrorist Radicalization Assessment Protocol (TRAP‐18) finds that 72% of the warning indicators were present, including four proximal warning behaviors suggesting high or imminent risk for a targeted attack: pathway, identification, last resort, and energy burst. The case also illustrates the impact of this attack on subsequent violent events through forensic linguistic analysis of Terrorgram and other lone actors' communications. The discussion focuses upon mitigation of such lone‐actor violence, including the implementation of behavioral threat assessment and management (BTAM) teams, the use of structured professional judgment instruments (SPJ) to assess risk, the simultaneous and continuous monitoring of both online and on‐the‐ground activity of the person of concern, the location and speed of movement on the pathway to violence, and disaggregation of the motivation, risk and potential outcome in the case through scenario planning and dynamic management. The authors close with a discussion of pertinent questions for altering the macro environment, particularly online, to mitigate the risk of future attacks.\n"]