Identifying pathways through which environmental risk factors influence PTSD is important for understanding PTSD etiology. Here, we hypothesized that the physical proximity to threat influences PTSD risk by increasing ASD following trauma. One hundred six bank employees who had experienced a bank robbery participated in the study. A longitudinal design assessing ASD at day 2 and PTSD at day 30 was used to test the hypothesis. Participants also indicated their location in the bank at the time of the robbery. ASD was identified in 40 (38%) and PTSD in 16 (15%) of the robbery victims. Distance to the robber had a strong effect on ASD (OR 3.51, 95% CI 1.94-6.34) and a somewhat lesser effect on PTSD (OR 2.15, 95% CI 1.04-4.46), indicating that the effect of proximity to threat on PTSD 1 month following trauma could be mediated by its effect on ASD 2 days following trauma. Using structural equation modeling, we confirmed that the effect of distance on PTSD was fully mediated by ASD. These findings suggest that proximity to threat may increase PTSD risk by enhancing the acute stress response following trauma.