This Essay investigates Chicago city-government policy responses to the four largest homicide waves in its history: 1920–1925, 1966–1970, 1987–1992, and 2016. Through spatial and historical methods, we discover that Chicago police and the mayor’s office misused data to advance agendas conceived prior to the start of the homicide waves. Specifically, in collaboration with mayors, the Chicago Police Department leveraged its monopoly over crime data to influence public narratives over homicide in ways that repeatedly (1) delegitimized Black social movements, (2) expanded policing, (3) framed homicide as an individual rather than systemic problem, and (4) exclusively credited police for homicide rate decreases. These findings suggest that efforts to improve violence-prevention policy in Chicago require not only a science of prevention and community flourishing but also efforts to democratize how the city uses data to define and explain homicide.