This paper explores the role of internal communication in one under-researched form of organizational crisis, insider threat – threat to an organization, its people or resources, from those who have legitimate access. In this case study, we examine a high security organization, drawing from in-depth interviews with management and employees concerning the organizational context and a real-life incident of insider threat. We identify the importance of three communication flows (top-down, bottom-up, and lateral) in explaining, and in this case, enabling, insider threat. Derived from this analysis, we draw implications for communication and security scholars, as well as practitioners, concerning: the impact of unintentional communication, the consequences of selective silence and the divergence in levels of shared understanding of security among different groups within an organization.