Methods, systems, and apparatus, including computer programs encoded on computer storage media, for fraud detection. One of the methods includes partitioning a feature space into a plurality of sub feature spaces, wherein the feature space comprises features associated with user account events for an online service; generating one or more clusters of users for each of one or more sub feature spaces; comparing a feature profile of one or more of the clusters with a global feature profile to determine features of one or more the clusters that have concentrated key values that exceed a respective threshold value; for each of the one or more clusters, scoring the cluster including aggregating the degree to which the key values for features exceed the corresponding threshold values; and based on the scores of the one or more clusters, determining one or more fraud detection actions.