Mexico has an impunity problem. Since the start of the War on Drugs in 2006, the incidence of serious crimes has increased dramatically. Despite enormous social pressure, conviction rates have declined. Why do Mexican prosecutorial institutions fail to punish crimes despite overwhelming desire for legal redress? This project traces the institutional origins of systematic impunity in Mexico through an ethnographic study of the routine encounters between prosecutorial officials and residents of a low-income neighborhood in Mexico City. By making these encounters its central object, this research contributes to theoretical discussions of criminalization, the negotiated character of the rule of law, and the influence of legal language on moral judgement. Preliminary fieldwork with prosecutors suggests that the ambiguity of criminal codes and the inconclusiveness of evidence in some cases rarely point towards an obvious outcome. Because of the open-ended character of the criminal procedure, discretion is a central aspect of prosecution. Nevertheless, rather than giving absolute power to prosecutors, discretion is one factor among many influencing the everyday negotiations of the rule of law. Rather than taking criminal investigation as a straightforward administrative procedure dictated by prosecutors, this project explores it as a complex encounter between victims, suspects, and prosecutors that advance diverging interpretations of legal codes according to contextual understandings of justice. To understand the fraught relation between law and justice, I will conduct participant observation, semi-structured interviews and semiotic analyses of legal documents in a criminal investigation unit and a low-income neighborhood with high rates of incarceration. Through this diverse set of ethnographic methods, this project explores why criminal prosecution in Mexico so often fails to dispense justice to victims and accused.