The first people to suspect someone is planning an act of terrorism or violent extremism are often those closest to them. Encouraging friends or family to report an “intimate” preparing to perpetrate violence is a strategy for preventing violent extremist or targeted mass violence. We conducted qualitative-quantitative interviews with 123 diverse U.S. community members to understand what influences their decisions to report potential violent extremist or targeted mass violence. We used hypothetical scenarios adapted from studies in Australia, Canada, and the United Kingdom. Factors influencing reporting decisions include fears of causing harm to the potential violent actor, self, family, or relationships; not knowing when and how to report; mistrust of law enforcement; access to mental health services; and perceptions that law enforcement lacks prevention capabilities. White and non-White participants were concerned about law enforcement causing harm. Participants would contact professionals such as mental health before involving law enforcement and Black-identified participants significantly preferred reporting to non-law enforcement persons, most of whom are not trained in responding to targeted violence. However, participants would eventually involve law enforcement if the situation required. They preferred reporting in-person or by telephone versus on-line. We found no difference by the type of violent extremism or between ideologically motivated and non-ideologically motivated violence. This study informs intimate bystander reporting programmes in the U.S. To improve reporting, U.S. policymakers should attend to how factors like police violence shape intimate bystander reporting. Our socio-ecological model also situates intimate bystander reporting beside other population-based approaches to violence prevention.