Using network analysis. A very nice example, with graphic visualization examples. See the link for details.
" .... Applying machine learning to AML ( Money laundering) has been challenging due to the limited availability of labeled datasets. However there are a number of unsupervised techniques that may be worth considering.
Network modeling is a powerful approach to AML analysis (Möser). Each account and real-world entity is set up as a node of a graph and transactions constitute the edges. Edges can have weights. Edge weights typically reflect the volume or the monetary value of transactions flowing between nodes. .... "