I believe in the circumstances spelled out in the original post, the continuing investigation would focus on intent of the human that brought the fuel and the heat source together. The use of the scientific method does not stop at the fire scene and not all data or information that needs to be analyzed is obtained at the fire scenein every instance. Even outside this human involvement scenario, one can continue to gather and analyse information, forumlate hypothesis and test hypothesis after leaving the fire scene. These continued actions may, in some instances, allow you to develop enough information to change your determination from undetermined to accidental or undetermined to intentional.

Thanks

Bobby