The relevance of the study is determined by the increasing number of terrorist attacks and the need to improve the efficiency of interagency cooperation as regulated under domestic legal provisions and international conventions during the investigation of murders committed for terrorist motives. The aim of the study is to analyse and improve mechanisms for interagency cooperation as regulated under domestic legal provisions and international conventions in the investigation of terrorist murders. The following methods were used in the study: relevance analysis, doctrinal comparative legal analysis, comparative cluster analysis, topological analysis, algorithm generalization, algorithm optimization, step-by-step structural qualitative comparative analysis, step-by-step structural element comparative analysis, and efficiency determination. The generalized results of the study showed that among the 25 analysed countries and international organizations, only 32% have highly effective integrated interaction models. The optimized algorithm reduces the operational complexity of the investigation process from 212 to 170 conventional units (≈20%), thereby increasing the speed of response, the accuracy of the services’ actions, and the ability to legal adaptation within different jurisdictions.