Arson or accident?

In 1985, at the Bradford City football ground, white smoke was seen emerging from beneath the terraced seating of the 77 year-old wooden building. Within two minutes the main stand was engulfed in flames. The speed of events was fast and 56 fans were trapped and died while around 200 were injured and others suffered from smoke inhalation. It was concluded that the fire was an accident and not arson. How can you tell the difference?