Over the past 20 years, there’s been a revolution in the science of arson investigations. Many of the clues that had been used for decades to determine that a fire was not accidental, especially the analysis of burn patterns on walls and floors, have been proven to be false. Reporter Michael May looks closely at two deadly fires to explore the cutting edge of fire science—including how a new understanding of a phenomenon called flashover has disproved much of the old assumptions, and how clues left in victims’ blood can help determine how a fire started and spread.