But Hurst concluded that it was a natural product of the dynamics of fire during post-flashover.
Firefighters using breathing apparatus practise in the unit to train against phenomena known as flashover and backdraught.
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It turned out that these classic signs of arson can also appear on their own, after flashover.
Subsequent tests by scientists showed that, during post-flashover, burning under beds and furniture was common, entire doors were consumed, and aluminum thresholds melted.
Arson investigators knew about the concept of flashover, but it was widely believed to take much longer to occur, especially without a liquid accelerant.
With the naked eye, it is impossible to distinguish between the pour patterns and puddle configurations caused by an accelerant and those caused naturally by post-flashover.
In a small fire, a V-shaped burn mark may pinpoint where a fire began, but during post-flashover these patterns can occur repeatedly, when various objects ignite.
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.
They are taught to look above and below seats and in lavatories and galleys, all the while watching the ceiling of the cabin to see if gasses are building that would create a flashover fire.
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In a scathing report, he concluded that investigators in the Willingham case had no scientific basis for claiming that the fire was arson, ignored evidence that contradicted their theory, had no comprehension of flashover and fire dynamics, relied on discredited folklore, and failed to eliminate potential accidental or alternative causes of the fire.
One of the fire investigators, who had been standing by an open door in the living room, escaped moments before the oxygen-starved fire roared out of the room into the hallway a fireball that caused the corridor to go quickly into flashover as well, propelling the fire out the front door and onto the porch.
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