Assuring that security requirements have been met in design phases is less expensive compared with changes after system development. Security-critical systems deployment requires providing security cases demonstrating whether the design adequately incorporates the security requirements. Building arguments and generating evidence to support the claims of an assurance case is of utmost importance and should be done using a rigorous mathematical basis, namely formal methods. In this paper, we propose an approach that uses formal methods to construct security assurance cases. This approach takes a list of security requirements as input and generates security cases to assess their fulfillment. Furthermore, we define security argument patterns supported by the formal verification results presented using the GSN pattern notation. The overall approach is validated through a case study involving an autonomous drone