This study examined the 15 deadliest public mass shootings in the United States from March 1998 to February 2018 to assess (a) leakage of violent thoughts/intent, (b) leakage of specific interest in mass killing, (c) concerning behaviors reported to law enforcement, (d) concerning interest in homicide reported to law enforcement, and (e) firearms acquisition. We then compared our findings on the deadliest public mass shooters with the Federal Bureau of Investigation’s (FBI) findings on active shooters in general. Overall, the results suggest that most incidents were indeed preventable based on information known about offenders in advance, and that the deadliest mass shooters exhibited more warning signs and were more often reported to law enforcement than other active shooters. Future prevention efforts should aim to educate, encourage, and pressure the public to report warning signs to law enforcement, educate and train law enforcement so that they can more effectively investigate potential threats, and limit firearms access for people who have admitted having homicidal or suicidal thoughts or being interested in committing a mass shooting. These relatively straightforward steps could significantly reduce the prevalence of future attacks.