Trauma victims often come to remember experiencing more trauma than they initially reported. Our experiments are the first to investigate a plausible mechanism for this memory amplification, namely, that people incorporate new details contained in post-event information (PEI) into their event memory. In Experiment 1, participants viewed traumatic photographs, and completed recognition memory tests before and 24-h after PEI exposure. As predicted, PEI distorted traumatic memory. In Experiments 2 and 3, we found that encouraging source monitoring reduced PEI-related errors regardless of whether the delay period between memory tests was shorter (24 h; Experiment 2) or longer (1 week; Experiment 3). Our results suggest that evaluating the source of traumatic photos increases source monitoring, making people resistant to inaccurate PEI.