This study compares foiled and completed mass shootings involving leakage (2000–2019) to determine the factors associated with leakage, modes of leakage, and bystanders encountering leakage that contributed to preventing mass shootings. Findings indicate offenders who engaged in leakage were more likely to be foiled when they were younger (< 18 years old), intended to target education locations, worked with co-offenders, and leaked information exclusively online. While not statistically significant, a higher proportion of mass shootings were also foiled after offenders leaked information to friends and classmates. Alternatively, offenders who engaged in leakage were more likely to complete their attack when they were middle-aged (35–44 years old); targeted commerce and workplace locations; leaked information via in-person communication and written statements; and leaked information to family members, wives/girlfriends, and coworkers. A discussion of findings illustrates the successes and failures in leakage-based prevention, as well as avenues for advancement in leakage identification and reporting.