Corner & Gill (2015) and Gruenewald, Chermak, & Freilich (2013) found that mass casualty offenders/ lone-actor terrorists are significantly more likely to have a mental disorder than group actors. Thirty-two percent of lone actors have evidence of mental illness, compared to 3 percent of group actors. The greater the isolation of the individual in terms of co-offenders/social networks, the greater the likelihood of mental illness.
In a study of the pre-attack behaviors of 63 active shooters (Silver, Simons, & Craun, 2018), the FBI found that 16 (25 percent) had a confirmed diagnosis of mental illness, including mood disorder, anxiety, psychosis, personality disorder and autism
Substance use disorders account for 34 percent of the risk of committing violence toward others. Overall, the best predictor of future violence is past violence (Elbogen & Johnson, 2009; Rozel, Jain, Mulvey, & Roth, 2017; Rozel & Mulvey, 2017).