Research into terrorism risk profiles has long been an integral part of the political science, psychology, and criminology fields. Before 9/11, debates on the importance of rational choice and/or personality were theoretical in nature. Al Qaeda's attacks moved this debate to the forefront of the domestic and international politics. An investigation into the September 11 attacks conducted by the United States (US) revealed that many of those involved in the attacks were considered foreign cell members operating in other liberal democracies (e.g., Germany), constituting a previously unknown threat profile (National Commission on Terrorist Attacks Upon the United States, 2004). As a result, the need to identify radicalized individuals became one of the priorities for liberal democratic governments. Although often successful at identifying radicalized individuals, national security agencies have struggled to determine who is at most risk of committing a terrorist attack. Unable to monitor every radicalized individual 24 hours a day, governments have placed trust into a variety of terrorism risk assessment instruments (TRAs). Most instruments currently in use focus heavily on the process of radicalization and extremist involvement while minimizing the importance of personality traits and disorders (Lloyd, 2019). As a result of a number of publicized false-negatives and the increased threat emerging from the lone actor spectrum, Corrado has developed a terrorism risk assessment instrument based on themes emerging from academic research, case studies, the Personality Disorder Inventory (PID) of the DSM-5, and the Comprehensive Assessment of Psychopathic Personality (CAPP) (Corrado et al., 2012). This dissertation assessed the strength of Corrado's TRA by conducting an exploratory four-part validation study. First, the study evaluated interrater reliability; second, the instrument's utility in predicting risk across ideologies was assessed; third the instrument was used to compare personality profiles of lone actor terrorists to group-based terrorists, and last, possible overlap in profiles of lone actor terrorists and mass shooters was examined. Findings suggest that there is no need for ideologically categorized terrorism risk assessment and that lone actors constitute a distinct terrorist typology.