The current research study aims to compare the effectiveness of two proven treatments for posttraumatic stress disorder (PTSD): Prolonged Exposure (PE), sertraline, and their combination. In addition, the investigators are examining predictors of response to these two treatments and how PTSD symptoms, thoughts, and biological factors may be changed by such treatments. Biological mechanisms of change are also examined including emotion processing and regulation in fMRI, HPA axis function, and genetics and genomics. In addition, the investigators will examine acceptability of each treatment and reasons for ending treatment.