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Preventing single actor extremist attacks: the TRAP-18

March 17, 2020  By Dr. Peter Collins and Dr. Alasdair Goodwill

Attacks at the hands of single-actor extremists are uncommon events but prove to be frightening and potentially devastating when they occur. It is extremely difficult for law enforcement to predict the actions of the single actor as they have inherently different motivations, different identified ideologies, as well as possible emotional and psychological disturbances that must be considered by threat assessment specialists.

Although there is no specific “profile” or “type” of single-actor extremist, we are getting better at understanding the behavioural, cognitive and emotional escalation of their path to violence – the series of sequential steps which indicate that an individual is progressing towards an act of targeted violence.1

However, according to Meloy & Hoffman (2014), accurately predicting the risk of an individual committing an act of targeted violence is highly unlikely due to their low incidence (i.e., low base-rate). Nevertheless, prevention is possible if the focus is upon evidence-based and factual behavioural indicators of targeted violence, and threat management is employed to mitigate these behaviours of concern.2

In this two-part article, we will present a brief summary of the Terrorist Radicalization Assessment Protocol, the TRAP-183, developed by Dr. Reid Meloy, a renowned forensic psychologist. The TRAP-18 is a structured professional judgment instrument composed of eight proximal warning behaviours (outlined here, in part one) and 10 distal characteristics (outlined in part two next month). Each indicator is coded as present, absent, or unknown (insufficient data to code).


Unlike other instruments that exist – the VERA (Pressman, 2009) and the ERG 22 + (Lloyd & Dean, 2015), the TRAP-18 was intentionally designed to assess for risk of targeted or intended extremist violence in community subjects — regardless of their particular ideology — to assist in prioritizing cases for investigators.

The proximal warning behaviours, the first eight indicators on the TRAP-18, have been shown to have validity in separating school attackers from other subjects of concern with no intent to attack.4 Proximal warning behaviors of pathway, identification, energy burst, and last resort were significantly more frequent among North American attackers than those that were identified as national security concerns but did not mount an attack5.

Proximal warning behaviours

  1. Pathway – research, planning, preparation or implementation of an attack.
  2. Fixation – an increasingly pathological preoccupation with a person or a cause, accompanied by a deterioration in social and occupational life. A fixation warning behaviour appears to have one of three cognitive-affective drivers: delusion, obsession, or extreme overvalued belief.6
  3. Identification – a psychological desire to be a pseudo-commando or have a warrior mentality, closely associate with weapons or other military or law enforcement paraphernalia, identifies with previous attackers or assassins, or identifies oneself as an agent to advance a particular cause or belief system.
  4. Novel Aggression – an act of violence that appears unrelated to any targeted violence pathway and is committed for the first time.
  5. Energy Burst – an increase in the frequency or variety of any noted activities related to the target, even if the activities themselves appear innocuous, usually in the hours, days or weeks before the attack.
  6. Leakage – communication to a third party of an intent to do harm to a target through an attack.
  7. Last Resort – evidence of a “violent action imperative” and “time imperative;” it is often a signal of desperation or distress.
  8. Directly Communicated Threat – the communication of a direct threat to the target or law enforcement beforehand. Although directly communicated threats are quite infrequent among those who engage in targeted violence of any kind (fewer than 20 per cent), they always warrant active investigation because they may turn out to be true positives. In a few cases, they are signaling an intent to act. Meloy and Gill (2016) found this to be the case in one out of five lone actor terrorists.7

As a cautionary note, full training is required to be able to use the TRAP-18 and the aforementioned is just a brief overview of the TRAP indicators. Many other articles are available that detail the research supporting the validity of the Terrorist Radicalization Assessment Protocol. Online training can be accessed through Dr. Meloy’s website is

Next month we will further explore the TRAP-18 and the distal characteristic indicators.


1. Calhoun, F.S. & Weston, S.W. (2003). Contemporary threat management: A guide for identifying, assessing, and managing individuals of violent intent. San Diego, CA: Specialized Training Services.
2. Meloy, J. R., & Hoffmann, J. (Eds.) (2014). International handbook of threat assessment. New York: Oxford University Press.
3. Meloy, J. R. (2017). The TRAP-18 manual version 1.0. Toronto: Multihealth Systems (
4. Meloy, J. R., Hoffmann, J., Roshdi, K., & Guldimann, A. (2014). Some warning behaviours discriminate between school shooters and other students of concern. Journal of Threat Assessment and Management, 1, 203–211.
5. Meloy, J. R., Goodwill, A. M., Meloy, M. J., Amat, G., Martinez, M., & Morgan, M. (2019). Some TRAP-18 indicators discriminate between terrorist attackers and other subjects of national security concern. Journal of Threat Assessment and Management.
6. Meloy, J. R., Roshdi, K., Glaz-Ocik, J., & Hoffmann, J. (2015). Investigating the individual terrorist in Europe. Journal of Threat Assessment and Management, 2, 140–152.
7. Meloy, J. R. & Gill, P. (2016). The lone actor terrorist and the TRAP-18. Journal of Threat Assessment and Management, 3, 37-52.

Dr. Peter Collins is the operational forensic psychiatrist with the Ontario Provincial Police’s Criminal Behaviour Analysis Unit. He is also a member of the crisis/hostage negotiation team of the Toronto Police Service Emergency Task Force. His clinical appointment is with the Centre for Addiction & Mental Health in Toronto, and he is an associate professor with the University of Toronto. Dr. Collins’ opinions are his own and may not reflect the opinion of the OPP, University of Toronto, CAMH or his mother. Contact him at

Dr. Alasdair Goodwill is a clinical and forensic psychologist with over 20 years combined experience in behavioural investigative analysis, threat assessment, forensic risk assessment, and therapeutic services. He has a clinical practice and forensic consultancy and is an associate professor at Ryerson University. Contact him at

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