AbstractAn open source sample of 111 lone actor terrorists from the United States and Europe were studied through the lens of the Terrorist Radicalization Assessment Protocol (TRAP-18). This investigative template consists of 8 proximal warning behaviors and 10 distal characteristics for active risk management or active monitoring, respectively, by national security threat assessors. Several aspects of criterion validity were tested in this known outcome sample. Seventy per cent of the terrorists were positive for at least half or more of the indicators. When the sample was divided into Islamic extremists, right wing extremists, and single issue terrorists, there were no significant differences across all eighteen indicators except for four. When the sample was divided according to successful vs. thwarted attackers, the successful attackers were significantly more fixated, creative and innovative, and failed to have a prior sexually intimate pair bond. They were significantly less likely to have displayed pathway warning behavior and be dependent on a virtual community of likeminded true believers. Effect sizes were small to medium (phi = 0.190-0.317). The TRAP-18 appears to have promise as an eventual structured professional judgment risk assessment instrument according to some of the individual terrorist content domains outlined by Monahan (2012, 2016).