The article discusses the Complex Event Modeling, Simulation, and Analysis (CEMSA) program of the U.S. Department of Homeland Security's Science and Technology Directorate, and describes it use in modeling an assault on U.S. transportation infrastructure that includes a cyber attack and the release of chlorine gas. The authors describe the CEMSA system and algorithms being developed for it, and demonstrate how CEMSA helps analysts evaluate models for complex events and arrive at efficient decision-making.