From the New Yorker:

After Hurst had reviewed Fogg and Vasquez’s list of more than twenty arson indicators, he believed that only one had any potential validity: the positive test for mineral spirits by the threshold of the front door. But why had the fire investigators obtained a positive reading only in that location? According to Fogg and Vasquez’s theory of the crime, Willingham had poured accelerant throughout the children’s bedroom and down the hallway. Officials had tested extensively in these areas—including where all the pour patterns and puddle configurations were—and turned up nothing. Jackson told me that he “never did understand why they weren’t able to recover” positive tests in these parts.
Hurst found it hard to imagine Willingham pouring accelerant on the front porch, where neighbors could have seen him. Scanning the files for clues, Hurst noticed a photograph of the porch taken before the fire, which had been entered into evidence. Sitting on the tiny porch was a charcoal grill. The porch was where the family barbecued. Court testimony from witnesses confirmed that there had been a grill, along with a container of lighter fluid, and that both had burned when the fire roared onto the porch during post-flashover. By the time Vasquez inspected the house, the grill had been removed from the porch, during cleanup. Though he cited the container of lighter fluid in his report, he made no mention of the grill. At the trial, he insisted that he had never been told of the grill’s earlier placement. Other authorities were aware of the grill but did not see its relevance. Hurst, however, was convinced that he had solved the mystery: when firefighters had blasted the porch with water, they had likely spread charcoal-lighter fluid from the melted container.

Bolded part: perhaps reasonable doubt, but not “scientific PROOF of innocence.”

Italicized part: unbelievably poor investigative work. Where was the grill?

Is Fire Marshall an elected official or political appointee in Corsicana? One would hope that the Fire Marshall would be hired based upon expertise.