Drug sample comparison is a process used by the French National police to identify drug distribution networks. The current approach is based on manual comparison done by forensic experts. In this article, we present our approach to acquire, formalise, and specify expert knowledge to improve the current process. For modelling the underlying knowledge we use an ontology coupled with logical rules. The different steps of our approach are designed to be reused in other application domains. The results obtained are explainable making them usable by experts in different fields.