1. Angel Rabasa, Robert D. Blackwill, Peter Chalk, Kim Cragin, Christine Fair, Brian A. Jackson, Michael Michael Jenkins, Seth G. Jones, Nathaniel Shestak, and Ashley J. Tellis, “Lessons of Mumbai,” RAND Corporation (2009). Available at http://www.rand.org/content/dam/rand/pubs/occasional_papers/2009/RAND_OP249.pdf (accessed 7 September 2014).

2. Brian Michael Jenkins, “The Threat of a Mumbai-style Terrorist Attack in the United States,” RAND Corporation (2013). Available at http://www.rand.org/content/dam/rand/pubs/testimonies/CT300/CT391/RAND_CT391.pdf (accessed 7 September 2014).

3. U.S. Department of Homeland Security (DHS), “Right Wing Extremism: Current Economic and Political Climate Fueling Resurgence in Radicalization and Recruitment,” Office of Intelligence and Analysis Assessment(2009). Retrieved from http://fas.org/irp/eprint/rightwing.pdf (accessed 7 September 2014).

4. Rodger Bates, “Dancing with Wolves: Today's Lone Wolf Terrorists,” The Journal of Public and Professional Sociology 4(2012), pp. 1–14.

5. DHS, “Right Wing Extremism.”

6. Edwin Bakker and Beatrice de Graaf, “Lone Wolves: How to Prevent this Phenomenon?” International Centre for Counter-Terrorism-The Hague (2010). Available at http://www.icct.nl/download/file/ICCT-Bakker-deGraaf-EM-Paper-Lone-Wolves.pdf (accessed 9 September 2014).

7. See Bakker and De Graaf, “Lone Wolves: How to Prevent this Phenomenon;” Bates, “Dancing with the Wolves;” and Peter Bergen and Bruce Hoffman, “Assessing the Terrorist Threat,” Bipartisan Policy Center (2010). Available at http://bipartisanpolicy.org/sites/default/files/NSPG%20Final%20Threat%20Assessment.pdf (accessed 9 September 2014).

8. Erik J. Dahl, “The Plots that Failed: Intelligence Lessons Learned from Unsuccessful Terrorist Attacks Against United States,” Studies in Conflict & Terrorism 34(2011), pp. 621–648.

9. U.S. Department of Homeland Security (DHS), “Active Shooter: How to Respond,” Office of Intelligence and Analysis Assessment, (2008). Available at http://www.dhs.gov/xlibrary/assets/active_shooter_booklet.pdf (accessed 7 September 2014).

10. See Grant Duwe, Mass Murder in the United States: A History (Jefferson, NC: McFarland & Company, 2007).

11. See Marissa A. Harrison and Thomas G. Bowers, “The Autogenic Massacre as a Maladaptive Response to Status Threat,” The Journal of Forensic Psychiatry & Psychology 21(2010), pp. 916–932.

12. Lisa Aitken, Piet Oosthuizen, Robin Emsley, and Soraya Seedat, “Mass Murders: Implications for Mental Health Professionals,” International Journal of Psychiatry in Medicine 38 (2008), pp. 261–269.

13. See John P. Blair, Terry Nichols, and John R. Curnutt, Active Shooter: Events and Responses (Boca Raton, FL: CRC Press, 2013); Richard Kelly, “Active Shooter: Recommendations and Analysis for Risk Mitigation,” New York Police Department, (2012). Available at http://www.nyc.gov/html/nypd/downloads/pdf/counterterrorism.pdf (accessed 1 October 2014); John P. Blair and Katherine Schweit, “A Study of Active Shooter Incidents, 2000–2013,” U.S. Department of Justice (2014).

14. See Jeff Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists,” Homicide Studies 15 (2011), pp. 177–203; Jeff Gruenewald and Willam A. Pridemore, “A Comparison of Ideologically-Motivated Homicides from the New Extremist Crime Database and homicides from the Supplementary Homicide Reports Using Multiple Imputation by Chained Equations to Handle Missing Values,” Journal of Quantitative Criminology 28 (2012), pp. 141–162.

15. Grant Duwe, “Patterns and Prevalence of Mass Murder in Twentieth-Century America,” Justice Quarterly 21 (2004), pp. 729–761.

16. James Allan Fox and Jack Levin, Extreme Killing: Understanding Serial and Mass Murder (Thousand Oaks, CA: Sage Publications, 2012).

17. See Matt Delisi and Aaron M. Scherer, “Multiple Homicide Offenders: Offense Characteristics, Social Correlates, and Criminal Careers,” Criminal Justice Behavior 33 (2006), pp. 367–391.

18. Paul E. Mullen, “The Autogenic (Self-Generated) Massacre,” Behavioral Sciences & the Law 22 (2004), pp. 311–323.

19. Ronald M. Holmes and Stephen T. Holmes, Mass Murder in the United States (Upper Saddle River, NJ: Prentice Hall, 2003).

20. See Paul Gill, John Horgan, and Paige Deckert, “Bombing Alone: Tracing the Motivations and Antecedent Behaviors of Lone-Actor Terrorists,” Journal of Forensic Science 59 (2014), pp. 425–435; Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists;” and Gruenewald and Pridemore, “A Comparison of Ideologically-Motivated Homicides.”

21. See Gill et al.,“Bombing Alone;” Jeff Gruenewald, Steven Chermak, and Joshua D. Freilich, “Far-Right Lone Wolf Homicides in the United States,” Studies in Conflict & Terrorism 12 (2014), pp. 65–91; Ramon Spaaij. “The Enigma of Lone Wolf Terrorism: An Assessment,” Studies in Conflict & Terrorism 33 (2010), pp. 854–870.

22. See Spaaij, “The Enigma of Lone Wolf Terrorism,” p. 863.

23. Adam Lankford, “Do Suicide Terrorist Exhibit Clinically Suicidal Risk Factors? A Review of Initial Evidence and Call for Future Research,” Aggression and Violent Behavior 15 (2010), pp. 334–340.

24. Adam Lankford, “A Comparative Analysis of Suicide Terrorists and Rampage, Workplace, and School Shooters in the United States from 1990 to 2010,” Homicide Studies 17 (2013), pp. 255–274.

25. Ibid.

26. Clark McCauley, Sophia Moskalenko, and Bejamin Van Son, “Characteristics of Lone-Wolf Violent Offenders: A Comparison of Assassins and School Attackers,” Perspectives on Terrorism 17 (2013), pp. 255–274.

27. Ibid.

28. Lankford, “A Comparative Analysis of Suicide Terrorists.”

29. Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists.”

30. Ibid.

31. Gruenewald and Pridemore, “A Comparison of Ideologically-Motivated Homicides.”

32. See Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists;” Gruenewald and Pridemore, “A Comparison of Ideologically-Motivated Homicides;” and Raffaello Pantucci, “A Typology of Lone Wolves: Preliminary Analysis of Lone Islamist Terrorists,” The International Centre for the Study of Radicalization and Political Violence, (2011). Available at http://icsr.info/wp-content/uploads/2012/10/1302002992ICSRPaper_ATypologyofLoneWolves_Pantucci.pdf (7 September 2014).

33. Jeff Gruenewald, Steve Chermak, and Joshua D. Freilich, “Distinguishing ‘Loner’ Attacks from other Domestic Extremist Violence,” Criminology & Public Policy 12 (2013), pp. 65–91.

34. Ibid.

35. Gruenewald et al., “Far-Right Lone Wolf Homicides in the United States.”

36. See Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists;” Gruenewald and Pridemore, “A Comparison of Ideologically-Motivated Homicides;” and Gill et al., “Bombing Alone.”

37. Lankford, “A Comparative Analysis of Suicide Terrorists.”

38. Gruenewald et al., “Distinguishing ‘Loner’ Attacks from other Domestic Extremist Violence.”

39. DHS, “Active Shooter: How to Respond,” p. 2.

40. See Joshua Freilich, Steve Chermak, Roberta Belli, Jeff Gruenewald, and William Parkin, “Introducing the United States Extremist Crime Database (ECDB),” Terrorism and Political Violence 26 (2014), pp. 372–384.

41. For instance, suspect A gets arrested after engaging in an active shooting event. He is not affiliated with any extremist groups; according to people that knew him he holds extreme anti-Black and antisemitic views. After being arrested, he does not deny that he is responsible for the shooting, but he denies being racist. At this point we have two pieces of evidence, one that support and counter the extremist association. Under the Strength of Association Scale, this individual would receive a score zero. Let us assume, we dig a little deeper and we find that, among all the victims, the shooter selectively picked only those that were Black and Jewish. Now there are two pieces of evidence for and one against the extremist association—there is a higher degree of certainty that the suspect is an extremist. Under the scale, this attacker would get a score of 1. If for instance, we obtain evidence that he visited extremist websites, the then increased score would reflect that new piece of pro-association evidence.

42. Kelly, “Active Shooter: Recommendations and Analysis for Risk Mitigation.”

43. Ibid.

44. Lankford, “A Comparative Analysis of Suicide Terrorists.”

45. McCauley et al., “Characteristics of Lone-Wolf Violent Offenders.”

46. See Gruenewald, “A Comparative Examination of Perpetrated by Far-Right Extremists;” Gruenewald and Pridemore, “A Comparison of Ideologically-Motivated Homicides.”

47. See Gill et al.,“Bombing Alone.”

48. Park Elliot Dietz, “Mass, Serial, and Sensational Homicides,” Bulletin of the New York Academy of Medicine 62 (1986), pp. 477–491.

49. Ibid.

50. Gruenewald et al., “Far-Right Lone Wolf Homicides in the United States.”

51. McCauley et al., “Characteristics of Lone-Wolf Violent Offenders.”