Bystanders—those who observe or come to know about potential wrongdoing—are often the best source of preattack intelligence, including indicators of intent and “warning” behaviors. They are the reason that some planned attacks are foiled before they occur. Numerous studies of targeted violence (e.g., mass shootings and school shootings) have demonstrated that peers and bystanders often have knowledge of an attacker’s intentions, concerning communication, and troubling behavior before the attack occurs. This chapter describes—with empirical support—why threat assessment professionals should consider bystanders; outlines a model for understanding bystander decision-making; reviews common barriers to bystander reporting; and suggests ways to mitigate those barriers, to engage bystanders at an individual level, and to improve reporting. The principal aim of threat assessment is to prevent (primarily) intentional acts of harm. When tragic incidents of planned violence occur, however, it is almost always uncovered “that someone knew something” about the attack before it happened. This happens because, as attack plans unfold, people in several different roles may know, or come to know, something about what is happening before harm occurs. The perpetrators know, and so might others, including targets, family members, friends, coworkers, or even casual observers.
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