Methods, systems, and apparatus, including computer programs encoded on computer storage media, for detecting malicious attacks. One of the methods includes generating a collection of hypergraphs representing user events across a collection of users; analyzing the collection of hypergraphs to determine a group of malicious user accounts or account activities satisfying a threshold confidence; using the group of malicious user accounts or account activities as training data for a machine learning system that generates one or more classifiers; and using the one or more generated classifiers to output additional malicious user accounts or account activities.