Lone Offender Terrorists and Mass Murderers in the United States between 1990 and 2013 (ICPSR 36314)

Version Date: Sep 29, 2020 View help for published

Principal Investigator(s): View help for Principal Investigator(s)
John G. Horgan, Georgia State University; Paul Gill, University College, London


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The study examines the behavioral underpinnings of three types of United States-based offenders: solo terrorists, lone-actor terrorists, and individuals who engage in mass casualty violence but lack an ideological motivation (aka mass murderers). In particular, the researchers compare the developmental, antecedent behavioral, and ideological factors that crystallize within the offender and are later expressed behaviorally via an attack.

The data address the following research questions: What are the similarities and differences between lone actor terrorists and solo mass murderers, in terms of sociodemographic characteristics and behavioral traits prior to, during, and after their attack? Can risk factors associated with delinquency and criminal offenses also help predict low-likelihood, high impact events such as lone actor terrorist attacks and mass murders? Do these factors remain the same over time?

To complete the research objectives, the researchers used an open source methodological design, relying solely on text materials available within the public domain. Data were coded and then statistically analyzed using bivariate and multivariate tests. The sample (n = 186) was limited to United States-based offenders who operated between 1990 and 2013 and were identifiable through publicly available materials.

Horgan, John G., and Gill, Paul. Lone Offender Terrorists and Mass Murderers in the United States between 1990 and 2013. Inter-university Consortium for Political and Social Research [distributor], 2020-09-29. https://doi.org/10.3886/ICPSR36314.v1

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United States Department of Justice. Office of Justice Programs. National Institute of Justice (2013-ZA-BX-0002)


Inter-university Consortium for Political and Social Research

1990 -- 2013
2014 -- 2015

The purpose of this study is to understand the behavior of lone actor terrorists and solo mass murderer attackers. Rather than use the terms 'terrorist' or 'offender' as dependent variables, the researchers disaggregate and compare socio-demographic, psychological and behavioral features between these two offender types.

The study relied on open source methodology, using sources already available in the public domain for raw data. Initial actor dictionaries for lone actors and mass murderers were compiled from academic literature on the respective topic area. Further names of lone actors were added through LexisNexis searches for English-language news, the Global Terrorism Database, and lists of convictions of terrorism-related offenses. Further names of mass murderers were added through database searches (Mother Jones, USA Today, Mayors Against Illegal Guns), LexisNexis searches, and FBI Uniform Crime Reports. Other resources included public record depositories and biographies of specific lone actors. Following data coding, the researchers used bivariate and multivariate tests to analyze the data.

All United States-based individuals who a) fit the study definition of lone actor terrorist or solo mass murderers (please refer to the User Guide for more details), b) were identifiable through publicly available records and sources, and c) operated between 1990 and 2013. The final sample contains 71 lone actors and 115 mass murderers (n = 186).

Longitudinal: Cohort / Event-based

All identifiable individuals in the United States who fit the study definition of lone actor terrorist or solo mass murderer from 1990 to 2013, and who were identifiable through publicly available sources.


Variables are broadly grouped based on factors prior to, during, and following the attack for each individual. Sociodemographic variables are also included.

  • Sociodemographic items: Gender, year of birth, relationship status, occupation, parental status, education, military experience, citizenship, religion, ideology, age/year of first terrorist activity, any arrests and/or criminal convictions
  • Pre-event items: Grievances and other contributing factors to individual's social and psychological state (e.g. having been harmed, humiliated, or wronged; history of substance abuse or mental illness; recent or chronic stress; family, relationship, or financial issues; social isolation; changes in ideology; changes in work/school performance), whether a tipping point occurred, leakage-related behaviors (e.g. verbalizing intent and desire to hurt others, if others were involved or aware of planning)
  • Event items: Nature of target and location, type of event, attack method(s), number of people injured or killed in the attack
  • Network-related activities: Attempts to recruit others, interacted with or joined a wider network offline or online, wrote public statements, if family or associates were also involved, read material about other offenders
  • Post-event items: If individual was killed in the event, claimed responsibility, felt remorse, possessed materials publicizing their actions

Not applicable




2020-09-29 ICPSR data undergo a confidentiality review and are altered when necessary to limit the risk of disclosure. ICPSR also routinely creates ready-to-go data files along with setups in the major statistical software formats as well as standard codebooks to accompany the data. In addition to these procedures, ICPSR performed the following processing steps for this data collection:

  • Checked for undocumented or out-of-range codes.


  • The public-use data files in this collection are available for access by the general public. Access does not require affiliation with an ICPSR member institution.