Abstract Being socially integrated is regarded as a protective factor enabling people to cope with adversity. The stress-buffering effect reflects an interaction between stress and a social coping resource factor on subsequent outcomes. This study, based on 2943 police officers, examines mental health outcomes among officers who responded to the 9/11 terrorist attack on the World Trade Center. The Wave 1 data collection took place between September 2003 and November 2004 with a follow-up study (Wave 2) conducted from November 2006 through December 2007. A moderated mediation model was specified that uses event exposure as a distal predictor, earlier stress response as a mediator, and later stress response as an outcome, and social integration as a moderator of this relationship. The mediation hypothesis was confirmed, and moderation occurred at two stages. First, there was a multiplicative relationship between exposure levels and social integration: The higher the exposure level, the more stress responses occur, but this effect was buffered by a high level of social integration. Second, Wave 1 stress interacted with social integration on Wave 2 stress: The more the police officers were socially integrated, the lower the Wave 2 stress, which happened in a synergistic manner. The findings contribute to the understanding of mediating and moderating mechanisms that result in health outcomes such as posttraumatic stress disorder or resilience.