I used to be a burn nurse, just adding this FWIW. When an accelerant is used, it usually causes a flash of flame. Breathing in the fire could easily burn the tissues of the nose and mouth, even if no accelerant was poured into them. If Jessica was inside the car when the fire started, I think she would have been overcome with smoke and lost consciousness before 98% of her body could burn, and wouldn't be able to get out of the vehicle. I have seen deep 3rd and 4th degree burns on the front of the body with no burns on the back or butt where they sat or were laying. I think Jessica was standing when an accelerant was poured onto her, outside her car. She caught fire and she breathed in the flash of flames. The soles of her feet were not burned because she was standing when it happened. The fire would burn out as soon as the accelerant/clothing burned away. She fell to the ground and was found there. Maybe the 911 caller did see her running on fire, but she was on the ground by the time the first responder arrived. If so, the perp had to be RIGHT THERE. I've seen severe burns come to the ER and still be talking. But not for long.
Sorry if this is too graphic.