Police, like other bureaucratic agencies, are responsible for collecting and disseminating policy-relevant data. Nonetheless, critical data, including killings by police, often go unreported. We argue that this is due in part to the limited oversight capacity of legislative bodies to whom police are accountable. Although many local assemblies lack the means for effective oversight, well-resourced state legislatures may induce transparency from state and substate agencies. This argument is evaluated in two studies of police transparency in the United States. First, we examine the compliance of 19,095 state, county, and municipal police agencies with official data requests over five decades, finding strong positive effects of state legislative capacity on transparency. Second, we examine the accuracy of transmitted data on killings by police, finding that lethality is systematically underreported in states with lower-capacity legislatures. Collectively, our study has implications for research on policing, legislatures, agency control, and analyses of government data.