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This paper proposes an approach to the safety verification of the source code of a software-intensive system. This approach centers upon the production of a document intended to ensure the inspectability, maintainability and repeatability of the source code safety verification. This document, called a "safety verification case", is intended to be a part of the overall system safety case. Although the approach was designed for large software-intensive real-time information systems, it may also be useful for other kinds of large software systems with safety-related functionality. The approach involves the construction of a rigorous argument that the source code is safe. The steps of the argument include simplifying the safety verification case structure by isolating the relevant details of the source code, and reducing the "semantic gap" between the source code and the system level hazards through a series of hierarchical refinement steps. Some of the steps in a process based on this approach may be partially automated with tool-based support. Current research and industry practices are reviewed in this paper for supporting tools and techniques.