--- - |2+ Research Summary A sentinel event review (SER) is a system‐based, multistakeholder review of an organizational error. The goal of an SER is to prevent similar errors from recurring in the future rather than identifying and punishing the responsible parties. In this article, we provide a detailed description of one of the first SERs conducted in an American police department—the review of the Lex Street Massacre investigation and prosecution, which resulted in the wrongful incarceration of four innocent men for 18 months. The results of the review suggest that SERs may help identify new systemic reforms for participating police departments and other criminal justice agencies. Policy Implications Police departments and other criminal justice agencies should begin implementing SERs to review a wide range of organizational errors and “near misses.” We offer guiding principles about the kinds of errors that may be more or less susceptible to fruitful review. Congress, state legislatures, and municipalities should also enact policies—such as safe harbor provisions—to encourage agencies to conduct SERs. - 'Criminology &Public Policy, Volume 18, Issue 3, Page 705-730, August 2019. '