Results found that while catastrophic events such as terrorist attacks may have an effect on individual and collective efficacy well beyond the immediate impact of the incidents, these effects are not strong enough to influence homicide rates. There is substantial evidence that catastrophic events, including terrorist attacks, lead to increased levels of post-traumatic stress, especially in communities in close proximity to the incident, and that some scholars also argue that these events disrupt social organization. On the other hand, many contend that these incidents produce social cohesion as community members coalesce to help each other in time of need, and that these ideas have resulted in competing hypotheses in existing literature. The first hypothesis examined is that violence will increase in the wake of catastrophic events due to heightened levels of individual stress and community disorganization. The second hypothesis examined is that violence will decline after these events because of increased social cohesion, especially in the face of an outside threat. In order to test these competing hypotheses, the work employed autoregressive integrated moving average (ARIMA) techniques to model the impact of the Oklahoma City bombing and the September 11 attacks on monthly homicide counts at the local, State, and national level. Unlike prior studies that provided evidence of an effect but did not use rigorous time-series techniques, the authors found no support for either of the competing hypotheses. Data were derived from local, State and national indices in New York and Oklahoma, along with data for United States homicide counts from the Centers for Disease Control and Prevention. Figures, tables, and references