The goal of this study was to examine whether panic mediates the relationship between fear, helplessness, and horror (PTSD criterion A2) and dissociation at the time of trauma. The study sample included 709 police officers and 317 peer-nominated civilians who had been exposed to a variety of critical incidents. Participants filled out measures of critical incident exposure, PTSD criterion A2, panic, and dissociation. Results indicate that together, physical and cognitive symptoms of panic completely mediate the relationship between criterion A2 and dissociation in civilians, and partially mediate that relationship in police. These results provide support for the idea that panic mediates the relationship between fear, helplessness, and horror (criterion A2) and dissociation at the time of trauma. The results also raise the possibility, however, that the mediational role of panic may be further moderated by additional variables.