Black and Asian people in the United Kingdom are more likely to be stopped and searched by police than White people. Following a panel of 36,000 searches by 1,100 police officers at a major English police force, we provide officer-specific measures of over-searching relative to two baselines: the ethnic composition of crime suspects officers interact with and the ethnic composition of the areas they patrol. We show that the vast majority of officers over-search ethnic minorities against both baselines. But we also find that the over-searching by individual officers cannot account for all of the over-representation of ethnic minorities in stop and search: over-patrolling of minority areas is also a key factor. Decomposing the overall search bias, we find that the over-representation of Asian people in stop and search is primarily accounted for by over-patrolling, while the over-representation of Black people is a combination of officer and patrol effects, with the larger contribution coming from biases of officers.