As it has been practiced for decades, arson investigations were more voodoo than science, and “arson sleuths” were often dabblers; “old-timers” who lay claim to a “a body of wisdom,” passed down from one old timer to the next arson investigator. The problem? An innocent individual, Cameron Todd Willingham, was “executed for the arson murder of his three young daughters,” in Texas, in 2004, based on this hocus-pocus.

Many arson investigators, it turned out, had only a high-school education. In most states, in order to be certified, investigators had to take a forty-hour course on fire investigation, and pass a written exam. Often, the bulk of an investigator’s training came on the job, learning from “old-timers” in the field, who passed down a body of wisdom about the telltale signs of arson, even though a study in 1977 warned that there was nothing in “the scientific literature to substantiate their validity.”

In 1992, the National Fire Protection Association, which promotes fire prevention and safety, published its first scientifically based guidelines to arson investigation. Still, many arson investigators believed that what they did was more an art than a science—a blend of experience and intuition. In 1997, the International Association of Arson Investigators filed a legal brief arguing that arson sleuths should not be bound by a 1993 Supreme Court decision requiring experts who testified at trials to adhere to the scientific method. What arson sleuths did, the brief claimed, was “less scientific.” By 2000, after the courts had rejected such claims, arson investigators increasingly recognized the scientific method, but there remained great variance in the field, with many practitioners still relying on the unverified techniques that had been used for generations. “People investigated fire largely with a flat-earth approach,” Hurst told me. “It looks like arson—therefore, it’s arson.” He went on, “My view is you have to have a scientific basis. Otherwise, it’s no different than witch-hunting.”

On September 7, 2009, the New Yorker’s David Grann wrote a lengthy expose, “Trial by Fire,” in which he asked, “Did Texas execute an innocent man?”

On March 9, this year, Maurice Possley, of The Marshall Project, all but confirmed that yes, Texas executed an innocent man.

Read the horror story of Cameron Todd Willingham, who “insisted upon his innocence in the deaths of his children and refused an offer to plead guilty in return for a life sentence.”

And the update.