On April 16, 2007, in the worst campus shooting incident in U.S. history, 49 students and faculty at Virginia Polytechnic Institute and State University (Virginia Tech) were shot, of whom 32 were killed. A cross-sectional survey of 4,639 Virginia Tech students was carried out the following summer/fall to assess PTSD symptoms using the Trauma Screening Questionnaire (TSQ). High levels of posttraumatic stress symptoms (probable PTSD) were experienced by 15.4% of respondents 3 to 4 months following the shooting. Exposure to trauma-related stressors varied greatly, from 64.5% unable to confirm the safety of friends to 9.1% who had a close friend killed. Odds ratios for stressors predicting high levels of posttraumatic stress symptoms were highest for losses (2.6–3.6; injury/death of someone close) and inability to confirm the safety of friends (2.5). Stressor effects were unrelated to age, gender, and race/ethnicity. The exposures that explained most of the cases of high posttraumatic stress symptoms were inability to confirm the safety of friends (30.7%); death of a (not close) friend (20.3%); and death of a close friend (10.1%). The importance of high-prevalence low-impact stressors resulted in a low concentration of probable cases of PTSD, making it difficult to target a small, highly exposed segment of students for mental health treatment outreach. The high density of student social networks will likely make this low concentration of probable PTSD a common feature of future college mass trauma incidents, requiring broad-based outreach to find students needing mental health treatment interventions.