Mass murder is rare, but the impact these events have is devastating. Identifying warning behaviors of mass shooters could prevent these events. The present study analyzed 27 manifestos from 23 mass shooters in the United States. The manifestos were qualitatively coded for the eight preattack warning behaviors of shooters proposed by Meloy et al. (2012). Six of the eight warning behaviors were identified in the 27 manifestos. These warning behaviors have only been examined retrospectively. Future research should examine the ability of these waring behaviors to identify a potential mass shooter before they attack.