Context: Cyber-physical systems (CPSs) demand justification of system safety and security, and component reliability. The increase in interconnectivity of CPSs raises concerns related to security. Assurance cases provide an explicit means for justifying/assessing confidence in system dependability with explicit and implicit references to design, safety, security and reliability artifacts. Fault Tree Analysis (FTA) and Attack Tree Analysis (ATA) are the most popular safety and security analysis techniques, which are integral part of Model-Based Dependability Assessment. Motivation: Conversely, the open and adaptive nature of CPSs, demands a paradigm shift from design-time to runtime system assurance. Although the Structured Assurance Case Metamodel (SACM) provides the foundations for runtime system security assurance, enabling the traceability between assurance cases and attack trees, which is part of Executable Digital Dependability Identities (EDDIs) of CPS components, it is still a barrier. Research Goal: In this paper, we introduce a model-driven methodology to support the synthesis of assurance cases from attack trees as a step towards demonstration of CPS security at runtime.