This prospective study examined risk and protective factors in the development of posttraumatic stress disorder (PTSD) in a sample of 83 police officers. Structured interviews were conducted in order to assess the most recent work-related traumatic event and establish diagnoses of acute stress disorder (ASD) and full or partial PTSD. Police officers were assessed between 5 and 15 days, and at 1 month, 3 months, and 12 months after the event. They also completed self-administered questionnaires assessing several potential predictors. Predictive analyses about the onset of PTSD were based on a 4-step nested random-effect linear regression. Overall, results showed that the modulation of PTSD symptomatology was associated with some pretraumatic (i.e., emotional coping strategies and number of children), peritraumatic (i.e., physical and emotional reactions and dissociation), and posttraumatic factors (i.e., ASD, depression symptoms, and seeking psychological help at the employee assistance program and at the police union between the event and Time 1). Clinical implications of these findings are discussed and key directions for future studies are proposed.