In the literature, the association between medium and long-term PTSD (post-traumatic stress disorder) after terrorist attack has rarely been described. The objective of our study was to identify the factors associated with PTSD in the medium and longer term among people exposed to a terrorist attack in France. We used data from a longitudinal survey of 123 terror-exposed people interviewed 6-10 (medium term) and 18-22 (long term) months after. Mental health was assessed by the Mini Neuropsychiatric Interview. PTSD in the medium term was associated with history of traumatic events, low levels of social support and severe peri-traumatic reactions, which were in turn associated with high levels of terror exposure. PTSD in the medium term was linked in turn to the presence of anxiety and depressive disorders, which was also linked to PTSD in the longer term. The factors leading to PTSD are different in the medium and long term. In order to improve future support for people exposed to distressing events, it is important to follow up people with intense peri-traumatic reactions, high levels of anxiety and depression and to measure reactions.