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Since the first trigatron spark gap was described by J. D. Craggs, M. E. Haine, and J. M. Meek [J. Inst. Electr. Eng. 93A, 963 (1946)], there has been controversy about the physical mechanism responsible for triggering the devices. In this letter we present experimental evidence that directly shows the sequence of physical events responsible for triggering in the gap we studied, and we present a model for trigatron triggering based on this information. We believe this model to be general and discuss it in light of existing literature. We briefly discuss the implications of the model for the engineering design of trigatron gaps.