Lone-Actor Violence and Threat Assessment: Risk Factors and Limits
Author: Ukrainian Psychological Hub · Published: October 11, 2026 · Editorial Policy
Lone-actor violence poses a difficult preventive question: how can institutions recognize a developing risk of targeted harm while protecting people whose beliefs, distress, or conduct resemble features found in past cases? Evidence-based threat assessment approaches this question through verifiable behavior, changing circumstances, competing explanations, and proportionate risk management. The aim is to evaluate and reduce a credible developing threat, with an explicit record of what is known and what remains uncertain.
The scientific boundary is important. A 2025 systematic review of violent-extremism risk tools found no included study demonstrating the prospective ability of these tools to forecast future violent extremist outcomes. Meanwhile, the FBI and U.S. Secret Service describe behavioral threat assessment as a structured prevention process. The research and operational guidance inform action without providing a reliable individual prediction formula.
What is lone-actor violence?
Lone-actor violence describes planned or attempted violence substantially initiated and operationally executed by an individual without direct command from an organized group. This is a description of how an act is organized. It does not imply that the individual has no relationships, online communities, ideological influences, or previous association with organizations. A person may operate alone during an attack while being influenced by social networks, digital content, personal grievances, or broader movements.
The terms lone-actor terrorism, lone-offender terrorism, targeted violence, public-figure attack, and mass attack appear in related research but specify different populations and outcomes. Terrorism adds legal or research criteria concerning ideological or political purpose. Mass attack is defined by an event threshold, which varies across sources. A threat against a particular individual may involve none of those categories. For every finding, ask who was studied, how the event was defined, and whether the study's findings apply to the current situation.
The FBI's study of lone-offender terrorism in the United States reviewed 52 attacks occurring between 1972 and 2015. Its purpose was to understand known cases, their surrounding circumstances, and possible opportunities for recognition. Its findings should not be imported automatically into an unrelated employee dispute, nonideological assault, or clinical referral.
What does lone-actor threat assessment mean?
Behavioral threat assessment is an organized process of identifying, investigating, evaluating, and managing concerns about potential targeted violence. It brings together information about relevant conduct, corroboration, context, temporal changes, situational opportunities, and potential interventions. The basic questions concern the credibility and development of a threat, what further information is needed, which explanations fit the facts, and what lawful steps may reduce danger.
The FBI's Making Prevention a Reality guide and the Secret Service's October 2024 law-enforcement guide offer multidisciplinary frameworks. An institution may coordinate behavioral analysis, security, mental-health expertise, and law enforcement, depending on its mandate and jurisdiction. A team should document information sources, alternative explanations, the reason for a concern, and what could change its assessment.
Psychological constructs, behavioral observations, clinical diagnoses, risk factors, protective factors, screening results, psychometric scores, forensic opinions, and legal findings describe different things. A reported grievance is a circumstance; a verified message is behavior; a clinical diagnosis is a clinical conclusion; a score reflects the application of an instrument; a detention or criminal conviction requires separate legal authority. No accumulation of labels bypasses those distinctions.
The evidence: important studies and their limits
FBI research on identified lone offenders
The FBI's 2019 lone-offender study examined backgrounds, motivation, social connections, communications, and preparation in a selected set of completed attacks. It provides descriptive evidence about cases already known to investigators. Its results do not show how frequently similar statements and circumstances occur among people who never attack. That missing denominator is decisive for individual prediction.
Secret Service findings on mass attacks
The National Threat Assessment Center's Mass Attacks in Public Spaces: 2016–2020 analyzed 173 incidents occurring in U.S. public and semi-public settings. The report documents diverse attacker circumstances, stressors, communications, and opportunities for people around them to notice concern. It emphasizes that no single profile identifies a mass attacker. Many pre-attack observations are also common among people who never become violent or fall within constitutionally protected activity.
The sample was selected because the incidents occurred, and the researchers looked backward. It cannot provide a base rate for future lone-actor violence among everyone who expresses anger, experiences a job loss, or posts controversial ideas. Its findings are best used to help professionals ask informed questions and design appropriate preventive pathways.
Peer-reviewed behavioral studies
Gill, Horgan, and Deckert's study of 119 lone-actor terrorists described heterogeneous background and antecedent patterns, with no uniform profile. In many of the known cases, other people were aware of relevant grievances, views, or intentions. Meloy and Gill's 2016 TRAP-18 study coded 111 known lone actors using a structured warning-behavior framework and reported some associations across subgroups and outcomes. Such studies add evidence about the features of selected cases; they do not establish a universal checklist for identifying future offenders.
The selection and hindsight problems
Retrospective research observes people after a plot or violent event became known. Researchers and investigators may then have access to communications, testimony, evidence, and interpretations unavailable to a community team beforehand. Media records are uneven, some cases are far better documented than others, and known attackers can receive intense scrutiny. Samples may overrepresent adult men, particular jurisdictions, and cases in which investigators located usable public records.
The practical error is reasoning backward from the event to its antecedents and then treating those antecedents as reliable forward predictions. Many ordinary people experience similar stressors or grievances and never approach violence. Sound analysis explicitly considers both outcome selection and the much larger population of noncases.
Risk-relevant factors: behavior, circumstances, and change
Grievance and perceived injustice
A grievance may concern humiliation, interpersonal rejection, employment loss, discrimination, ideological conflict, or a perceived wrong. Grievances are widespread human experiences. Their existence does not establish violent intent, dangerousness, or criminality. Concern becomes more specific when credible facts indicate that a person connects a grievance to an intention or developing effort to injure a selected target.
Assessors examine the content, direction, and evolution of a grievance; relevant relationships; whether nonviolent resolution remains available; and whether claims about imminent harm are supported by verified actions. A complaint, lawsuit, petition, protest, or strong political criticism can be entirely lawful and nonviolent.
Fixation and identification
In the targeted-violence literature, fixation describes a persistent, intense focus on an individual, cause, or perceived enemy, especially when the preoccupation changes in troubling ways. Identification may describe increasing self-identification with a violent role or narrative. These are interpretive constructs about thought and behavior. They are not psychiatric diagnoses or stand-alone proofs of future violence.
The TRAP-18 evidence review discusses such warning behaviors, but does not establish a universal threshold that separates harmless preoccupation from impending attack. The context and change over time are central, and alternative explanations must be assessed.
Leakage and communications
Leakage is communication to someone other than the intended target suggesting a possible interest in or intention toward violence. It may occur in conversation, writing, messages, or digital posts. The question is whether the communication conveys credible information about a specific developing danger when read in full context. Meaning depends on the speaker, wording, audience, circumstances, corroboration, and whether behavior changes afterward.
Fiction, humor, quotation, political hyperbole, indirect grievance, and genuine communication of violent intent cannot be treated as interchangeable. Conversely, the absence of a direct threat does not guarantee safety. The FBI prevention resources stress the value of communicating behaviorally specific concerns rather than attempting amateur diagnosis or profiling.
Planning, preparation, and opportunity
Evidence that a person is purposefully moving toward a specific harmful act can be more situationally relevant than demographic or personality characteristics. A professional inquiry evaluates a documented sequence of relevant behavior, apparent purpose, capability, opportunity, changing constraints, and competing innocent explanations. It also distinguishes allegations from corroborated observations and respects applicable investigative and privacy laws.
The possession of lawful items, interest in an historical event, technical knowledge, or an unusual hobby has many ordinary explanations. Any inference of attack preparation needs a supported connection to a particular concern. A threat assessment report should reveal its reasoning rather than infer dangerousness from a cluster of ambiguous traits.
Social isolation and digital networks
The lone-actor label does not mean a person lives in complete isolation. Research finds substantial diversity in relationships and communication. Social withdrawal may be important to understanding a particular person's needs or circumstances, but it is neither a general cause of lone-actor violence nor a valid universal screening rule.
Digital spaces can expose people to violence-promoting narratives, reinforce grievances, or connect them with supportive, nonviolent resources. An account's content, provenance, and relation to conduct matter more than membership in an online category. Assessors should guard against false attribution, selective screenshots, and the assumption that consuming a particular genre of content proves violent intent.
Stressors, deterioration, and behavioral change
Known attacks sometimes follow personal losses, workplace conflicts, relationship changes, legal problems, or other stressors. These events are common across the population, including among peaceful people. Descriptive associations in attack samples are not causal demonstrations or reliable individual forecasts. Their relevance increases or decreases according to accompanying conduct and the specific situation.
A defensible assessment examines what changed, when it changed, how sources corroborate that change, whether concerns are intensifying or resolving, and which protective resources remain available. The Secret Service research provides descriptions of stressors in identified attacks; it does not justify treating every stressed person as dangerous.
Ideology and violent mobilization
A person's political, religious, moral, or philosophical ideas are not substitutes for behavioral evidence. The distinction between beliefs and credible movement toward unlawful violence is both empirically and legally important. Strong opinions, unpopular expression, and group identification may be protected by law, while actual threats or criminal conduct are assessed using the standards that apply in the jurisdiction.
Motives may mix personal grievances, ideology, social recognition, perceived injustice, and other considerations. Professionals should document evidence of conduct and intent without assuming that one ideological label offers a complete psychological explanation.
Protective factors and opportunities for prevention
Protective factors may include reliable supportive relationships, effective nonviolent conflict resolution, readiness to accept appropriate help, stable practical supports, verified disengagement from a violent goal, and changes that reduce a specific situational hazard. Their meaning depends on the person and context. A reassuring circumstance should not be counted mechanically as proof that a threat has disappeared, and the absence of a presumed protective factor cannot by itself justify coercive action.
Risk management links potential intervention to the case formulation. Voluntary support, mediation when safe, clinical treatment for clinical needs, adjustments to conflict, appropriate security measures, and authorized law-enforcement action are possibilities in different circumstances. Their suitability requires factual support, attention to affected people, and lawful authority. A detailed related resource is Protective Factors in Violence Risk Assessment and Risk Management.
Assessment tools: TRAP-18, VERA-2R, and structured judgment
What is structured professional judgment?
Structured professional judgment is a method for organizing relevant factors, considering evidence and alternative explanations, and making an explicit formulation to support a decision. Unlike an automatically interpreted numerical forecast, the professional judgment must account for context, uncertainty, and the intended management action. The quality of implementation depends on training, reliable data, transparency, and a clearly defined population and outcome.
A structured framework can reduce omissions without validating a specific score as a probability of terrorism. Checklists, clinical judgments, actuarial calculations, and legal decisions have different purposes. Readers can compare these approaches in Actuarial vs. Structured Professional Judgment in Violence Risk Assessment.
What does TRAP-18 measure?
The Terrorist Radicalization Assessment Protocol contains eight proximal warning behaviors and ten distal characteristics intended to structure assessment of persons of concern for targeted ideological violence. Its categories help investigators organize potentially relevant patterns and their timing. It is not a psychiatric test, a legal definition of extremist behavior, a finding of criminal intent, or an established universal predictor of individual attacks.
Guldimann and Meloy's 2020 review discusses reliability and validity evidence for TRAP-18. The 2016 lone-actor study examined known outcomes retrospectively. Neither source demonstrates a general-population probability model calibrated to future violent events. Instrument creators' professional experience and early validation evidence are useful, but independent replication and prospective research remain crucial.
What about VERA-2R and related instruments?
The Violent Extremism Risk Assessment framework, Extremism Risk Guidance, Multi-Level Guidelines, and other violent-extremism instruments target partially different contexts and populations. Some have been used in correctional or specialist prevention settings. Their constructs and supporting studies should be evaluated individually. A method studied in a selected justice population cannot automatically be repurposed for broad screening of students, employees, or online users.
The 2025 Campbell systematic review assessed 20 studies involving six instruments. It found useful indications of research-setting interrater agreement, less consistent findings in field settings, sparse evidence across several validity domains, and significant study-design limitations. Its central conclusion is that none of the included studies prospectively validated prediction of future violent extremist outcomes. The review does not justify ranking one instrument as an accurate all-purpose predictor.
Why general violence tools have different validity
Broader violence risk instruments may be evaluated using general assault, institutional violence, or recidivism outcomes in clinical or correctional cohorts. Lone-actor terrorism and other rare targeted attacks are different outcomes with different base rates, surveillance patterns, and decision contexts. An instrument's published predictive discrimination in one group is not transferable without dedicated research.
The separate article Violence Risk Assessment: What It Can Predict and Where It Fails covers the general evidence on violence prediction, while this article retains the specific lone-actor targeted-violence intent.
Predictive validity, postdiction, and calibration
Predictive validity measures whether an assessment recorded before an outcome can forecast that outcome in an appropriate independent population. Postdiction examines patterns after an event's occurrence is known. Interrater reliability measures agreement among evaluators, which by itself says nothing about the correctness of a prediction. Discrimination measures ranking of cases with and without outcomes, while calibration compares estimated probabilities with observed event frequency. These measures are related but answer different questions.
A low base rate profoundly changes what a positive screen can mean. For a purely hypothetical example, imagine a population of 10,000 people with ten specified future events. A hypothetical rule with 80% sensitivity and 90% specificity would identify about eight of the ten future cases but incorrectly flag about 999 people who would not experience the outcome. Roughly eight out of 1,007 positive flags would be true positives. These numbers are an illustration of the mathematics of rare-event screening, not measured performance figures for TRAP-18, VERA-2R, or any real lone-actor tool.
False positives can lead to surveillance, damaged reputations, exclusion, employment consequences, and infringement of rights. False negatives can mean that a credible threat was not recognized. The effects depend on what the organization does with an assessment, whether corrective review is possible, the actual prevalence of the outcome, and the reliability of the data. A risk category cannot eliminate uncertainty, and neither impressive retrospective correlations nor interrater agreement prove reliable future identification.
Sample and population limitations
Much of the relevant literature draws on known male offenders, selected public records, specific ideologies, specialized settings, and Western jurisdictions. Some studies code events retrospectively from news or public documents. Such samples do not establish the distribution of risk among the population at large. Findings may vary with age, language, gender, cultural context, reporting practices, and the type of threat being investigated.
Independent testing should specify relevant outcomes and time horizons, assess false-positive and false-negative rates, evaluate calibration and subgroup variation, and test operational use. The 2025 systematic review identifies prospective validation and stronger methods as major needs, especially before conclusions affect consequential decisions.
Fairness, bias, and protected expression
Bias may enter at referral, information gathering, interpretation, tool coding, intervention, or follow-up. Differential surveillance can produce apparent differences in risk-related records across communities without corresponding differences in actual offending. Teams should ask whether comparable behavior is evaluated comparably, what evidence has been independently confirmed, and whether a person has a feasible process for correcting inaccuracies.
In the United States, First Amendment protections are central to evaluating speech, belief, and assembly. Relevant legal standards for threats, searches, disclosure, protective measures, and criminal proceedings vary by jurisdiction. Psychologists can describe behavioral observations and uncertainty; they do not decide constitutionality or criminal liability. Political identity, religious affiliation, and mental-health diagnosis are inappropriate shortcuts for evidence of a specific developing threat.
Mental illness, lone actors, and stigma
Lone-actor violence is a behavioral and investigative category, not a clinical diagnosis. Schizophrenia, depression, trauma-related conditions, autism, personality disorders, and other diagnoses do not establish violent intent or criminality. The majority of people living with mental-health conditions do not engage in serious violence. Clinical care should respond to clinical needs; threat assessment independently examines conduct relevant to possible harm.
Corner and Gill's 2015 study found differences in recorded mental-health histories between selected lone-actor and group-based terrorist samples. Its results cannot be applied to estimate the risk associated with a diagnosed individual in the general population, and diagnostic evidence within historical cases can be incomplete. The distinction between clinical symptom, circumstance, criminal behavior, and legal responsibility must be maintained at every stage.
A person may have distressing thoughts or express unsettling material without any intention to harm. A person can also plan violence without a known psychiatric disorder. Clinicians should avoid speculative diagnosis from a profile or online record; threat managers should not interpret a diagnosis as a standalone violence predictor. For disorder-specific evidence, see Schizophrenia and Violence: What Research Really Shows.
How a multidisciplinary assessment process works
1. Define and document the concern
A useful referral records observable conduct, dates, original evidence where lawfully available, sources, context, and the reason the report may relate to targeted harm. Words such as unusual, angry, isolated, radical, or unstable are impressions rather than sufficient grounds for a final risk conclusion. Institutions should define transparent intake thresholds and distinguish urgent threats from requests for clinical support or conflict management.
2. Gather and corroborate facts
Review firsthand and indirect reports, relevant communication in context, chronology, and source reliability. Document contradictions and missing information. A screenshot can be altered or misattributed, a quotation misunderstood, and a rumor repeated until it appears corroborated. Independent verification and alternative explanations are essential.
3. Formulate the case
A case formulation explains how available facts might relate to a specific possible threat, the relative strength of those facts, changes over time, protective considerations, and which additional findings would meaningfully change the assessment. It also names the decision the assessment is intended to support. A judgment such as low, medium, or high risk needs an operational definition and cannot be mistaken for a precise individualized probability.
4. Manage proportionately
Select interventions that address the evidence-supported concern and correspond to the institution's lawful role. Possible actions include further fact-finding, voluntary help, conflict resolution, protection for identified people, appropriate practical safeguards, or law-enforcement review if the applicable threshold is met. The Secret Service's 2024 guide provides an operational model for developing units and multidisciplinary cooperation; it does not create additional legal authority.
5. Reassess, close, and learn
Circumstances change. Document what would prompt renewed review, what evidence would lower or increase concern, the rationale for continued action, and criteria for case closure. Review false alarms and missed warning opportunities without treating every later incident as proof that the original assessment was defective. Assessments should remain revisable professional analyses rather than permanent labels imposed on a person.
How the setting changes the assessment
School systems must consider student development, educational obligations, family involvement, support services, and privacy. The U.S. Department of Education's FERPA guidance on outside threat assessment team members identifies conditions under which education records may be disclosed to qualifying outside team members and limits subsequent disclosure. This U.S.-specific rule cannot be generalized to every country.
Workplaces face different questions involving employee safety, harassment, access restrictions, employment rules, accommodations, and duty of care. Law-enforcement agencies may have additional investigative powers but must comply with relevant constitutional and statutory restrictions. Correctional settings may possess far more documented history than community organizations while studying different risk outcomes and populations. These contexts call for separately validated methods and tailored governance.
Forensic opinion is not a legal finding
A professional opinion about a possible developing threat is a structured interpretation of information; it is not a criminal conviction, proof of culpability, a determination of legal capacity, an insanity finding, or a judicial authorization for confinement. Laws governing threats, detention, evidence, privacy, reporting, and emergency response differ by country, and within the United States across federal and state systems.
Where a psychological opinion enters a legal proceeding, the evaluator should state the methods, population limits, available evidence, alternative interpretations, foreseeable errors, uncertainties, and scope of expertise. The American Psychological Association's Specialty Guidelines for Forensic Psychology emphasize accuracy, impartiality, competent methods, and fairness. Expert testimony and legal admissibility remain separate questions governed by the relevant court.
Ethical safeguards and practical implications
Sound programs establish who may refer cases, access sensitive information, evaluate evidence, contact other agencies, authorize action, correct records, and oversee outcomes. They need legitimate purposes for data collection, appropriate retention rules, accountability for intrusive measures, and review mechanisms proportionate to the consequences. The goal is to make prevention more effective while minimizing stigma and unjustified restriction.
Public reporting should be factual and narrow. If a person encounters a credible imminent threat to life, contacting emergency services is appropriate. For a non-immediate concern, established institutional or local law-enforcement reporting channels can assess specific observed behavior. Avoid public accusations, online campaigns, amateur diagnosis, or assumptions that an unusual belief identifies an attacker. A report starts a fact-finding process rather than establishing guilt.
What is established, preliminary, and contested?
Established operational principles include behaviorally specific inquiry, corroboration, review of alternative explanations, attention to changing circumstances, proportionate management, and protection of legal rights. FBI guidance and Secret Service guidance support multidisciplinary practice informed by incident studies and investigative experience. Those guides should be understood as professional resources, not prospective trials proving a precise prevention effect.
Preliminary evidence concerns the utility of specific warning constructs and structured protocols for describing selected known cases. Contested or unsupported claims include the ability to identify future lone-actor offenders by personality or demographic profile, prediction from diagnosis, a general-population risk percentage from TRAP-18, or transfer of accuracy statistics from unrelated violence instruments. The 2025 review sets out the current validation gap clearly.
Common myths about lone-actor threat assessment
A lone actor must be socially isolated: the term describes the operational organization of an act, and relevant relationships can exist.
An alarming belief or grievance predicts violence: such beliefs and experiences are common and require context and behavioral corroboration.
A direct threat always appears beforehand: communications differ, and an absence of one particular type of threat does not establish safety.
A risk instrument can determine who will attack: current research does not establish dependable prospective individual forecasts of rare lone-actor outcomes across general populations.
A mental-health diagnosis explains the act: diagnosis and criminal behavior are different matters; each requires separate evidence.
Threat assessment means arrest or surveillance: appropriate interventions may instead emphasize assistance, safe conflict management, factual inquiry, or lawful protective planning.
Frequently asked questions
Can lone-actor violence be predicted accurately?
Current research does not establish a validated method for accurately identifying future lone-actor attackers in the general population. Threat assessment can investigate credible evidence of a developing threat and support prevention. Recognition of a concern and a calibrated individual prediction are different claims.
Is lone-actor terrorism the same as mass shooting?
No. Lone-actor concerns operational independence, while mass shooting refers to an event defined by firearms use and a source-specific threshold. Some events may satisfy both categories, while others do not.
Is lone-actor terrorism different from lone-actor violence?
Lone-actor terrorism adds an ideological or legally defined terrorism component. Lone-actor violence is broader and may include nonideological or mixed-motive attacks. Definitions differ across studies and jurisdictions.
What is leakage in threat assessment?
Leakage is a communication to someone other than the intended target that may reveal thoughts or intentions related to an attack. Its evidentiary value depends on full context, corroboration, credibility, specificity, and temporal development.
Is fixation a diagnosis?
No. In this research, fixation is an interpretive description of intense preoccupation. It is neither a diagnosis nor a deterministic sign that someone will become violent.
Does TRAP-18 predict future terrorism?
Research shows selected retrospective associations, but the 2025 systematic review found no included prospective study establishing valid forward prediction of violent extremist outcomes. It should be interpreted as a structured professional aid with important limits.
What are false positives and false negatives?
A false positive is a case flagged as having a specified outcome that does not occur during the defined follow-up. A false negative is a case in which the outcome occurs despite an unflagged assessment. Their rates depend on the outcome, population, time horizon, decision threshold, and measurement design.
Can mental illness identify dangerous lone actors?
Clinical diagnosis alone does not determine violent intent or future criminal conduct. Clinical needs should be evaluated on clinical grounds, while threats are evaluated from reliable conduct-related evidence.
What is a protective factor?
A protective factor is a circumstance or capacity that may reduce risk in a defined context, such as credible engagement with support or nonviolent ways to address a grievance. No single protective factor guarantees safety.
Does ideological speech justify threat intervention?
Speech and belief require careful legal analysis. In the United States, the First Amendment protects many forms of expression; any institutional or legal response must rest on its applicable authority and the specific facts. Advocacy, unpopular views, and verified threatening conduct require careful distinctions.
Should a school or employer use a single risk score?
A single score cannot substitute for a documented, multidisciplinary assessment of the evidence, applicable law, competing explanations, possible error, and proportionate action. School privacy and employment rules impose additional obligations.
What should a witness do if concerned about a credible threat?
For immediate danger, contact local emergency services. Otherwise, use established institutional or law-enforcement reporting routes and share specific verifiable observations. Avoid personal confrontation, public speculation, and attempts to diagnose someone remotely.
Related articles
For general measurement and forecasting limits, read Violence Risk Assessment: What It Can Predict and Where It Fails. The method comparison is covered in Actuarial vs. Structured Professional Judgment in Violence Risk Assessment. For base rates, calibration, and reoffending outcomes, see Risk Assessment and Recidivism: Prediction, Calibration, and False Positives. Risk management is developed in Protective Factors in Violence Risk Assessment and Risk Management. For professional context, see Forensic Psychology: What It Is and What Forensic Psychologists Do.
Related Articles
References
American Psychological Association. (2013). Specialty guidelines for forensic psychology. APA professional guideline.
Brouillette-Alarie, S., et al. (2025). Reliability and validity of risk assessment tools for violent extremism: A systematic review. Campbell Systematic Reviews. https://doi.org/10.1002/cl2.70080.
Corner, E., & Gill, P. (2015). A false dichotomy? Mental illness and lone-actor terrorism. Law and Human Behavior, 39(1), 23–34. https://doi.org/10.1037/lhb0000102.
Federal Bureau of Investigation. (2017). Making prevention a reality: Identifying, assessing, and managing the threat of targeted attacks. FBI publication.
Federal Bureau of Investigation. (2019). Lone offender: A study of lone offender terrorism in the United States (1972–2015). Department of Justice resource.
Federal Bureau of Investigation. Behavioral analysis and prevention resources. FBI official resource.
Gill, P., Horgan, J., & Deckert, P. (2014). Bombing alone: Tracing the motivations and antecedent behaviors of lone-actor terrorists. Journal of Forensic Sciences, 59(2), 425–435. https://doi.org/10.1111/1556-4029.12312.
Guldimann, A., & Meloy, J. R. (2020). Assessing the threat of lone-actor terrorism: The reliability and validity of the TRAP-18. Forensische Psychiatrie, Psychologie, Kriminologie, 14, 158–166. https://doi.org/10.1007/s11757-020-00596-y.
Library of Congress. Overview of freedom of the press and speech. Constitution Annotated. Congress.gov.
Meloy, J. R., & Gill, P. (2016). The lone-actor terrorist and the TRAP-18. Journal of Threat Assessment and Management, 3(1), 37–52. https://doi.org/10.1037/tam0000061.
National Threat Assessment Center. (2023). Mass attacks in public spaces: 2016–2020. U.S. Secret Service. Official report.
National Threat Assessment Center. (2024). Behavioral threat assessment units: A guide for state and local law enforcement to prevent targeted violence. U.S. Secret Service. Official guide.
U.S. Department of Education. Does FERPA permit the sharing of education records with outside members of a threat assessment team? Protecting Student Privacy.