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.