The interdiction and criminal prosecution of a subject who arguably posed a risk of terrorism is studied through the lens of the Terrorist Radicalization Assessment Protocol (TRAP-18). Christopher Hasson, an active member of the U.S. Coast Guard, was arrested and prosecuted in light of his extremist activities and was found guilty on multiple federal drug and gun charges in 2019. He expressed strong white supremacist, neo-Nazi, and other far-right ideology in his writings and online research, admired infamous mass murderers, created a list of potential targets, and accumulated weapons while suffering from an opioid use disorder. An estimate of actual risk can be determined through an open-source intelligence analysis of his pre-arrest proximal warning behaviors and distal characteristics when compared to other samples of lone actor terrorists. We coded Mr. Hasson for 5 of 8 proximal warning behaviors and 7 of 10 distal characteristics on the TRAP-18, many of which are the most commonly present indicators in successful attackers across North America and Europe. A formulation of the case is then dependent on the individualizing of risk through a determination of the relevance and weight of the indicators in any one case. Such a formulation, guided by questions following the coding of the case, helps determine both the tactics and long-term strategy for management. The preventive nature of this case provides an opportunity to test the ecological validity of the TRAP-18 for use by counterterrorism professionals who must make such interdiction judgments-both their timing and nature-in every case they encounter. Public Significance StatementThe TRAP-18, a threat assessment tool, was applied to a case where a potential domestic terrorist was interdicted before he was able to commit an attack. Applying the TRAP-18 in a situation where it is unknown whether an attack would or would not have occurred provides further evidence of the tool's validity and its usefulness in assessing people of concern.