This paper presents a new data series for homicides of law enforcement officers. Available for more than two centuries, it is much longer than series previously examined. Police killings had two extreme peaks, one in the 1920s and another in the 1970s. We use the post-1930 part of the series in a time-series regression to explore structural conditions that affect police killings in the short term. Economic conditions, prison populations, and World War II have considerably larger impacts on police killings than on homicide generally. Police killings are less affected by demographic changes and by the crack epidemic.