Arson convictions are more difficult to overturn than cases where clear-cut DNA evidence points to a defendant's innocence. While it's likely that bad arson science was used in many cases, it's unclear how often that testimony made the difference in sending someone to prison, said James Doyle, director of the Center for Modern Forensic Practice at John Jay College of Criminal Justice.
"You're not going to have the comfort of an authoritative DNA test to tell you you're right or wrong," said Doyle, who is leading the first comprehensive study of old arson cases.
Despite retraining efforts and widely publicized cases like Dougherty's, some fire investigators continue to use and defend outdated techniques, Doyle and others say.
In training exercises for veteran fire investigators conducted by the federal government, fewer than a quarter of investigators generally were able to identify the cause of test fires, said Steve Carman, who recently retired as a senior special agent at the Bureau of Alcohol, Tobacco, Firearms and Explosives.
In one 2005 training, Carman found that less than 6 percent were able to identify the section of a room in which a test fire started.
"We're still finding that in some cases, depending on how the fire burns, some people still don't understand the things they're seeing," he said.
Advocates fear that outdated investigative techniques may have led to the execution of an innocent man. Last week the Texas Forensic Science Commission, created to look into allegations of forensic misconduct, announced that it would review the case of Cameron Todd Willingham, who was executed in 2004 for setting a fire that killed his three children.
A 2006 study sponsored by the Innocence Project concluded that the 1991 fire was not intentionally set. According to the study, the fire investigator's trial testimony was based on false premises.
Until the early 1990s, many fire investigators based their conclusions on what DeHaan calls the "cookbook approach" -- a series of telltale signs that they were taught pointed to arson, but were not subjected to rigorous scientific analysis.
Investigators were taught, for example, that finely cracked glass is caused by unusually hot fire, that fires always burn up, that fires caused by accelerants such as gasoline burn hotter than normal fires and that cracked concrete is a sign of an accelerant. All of those theories proved to be wrong.
"Fire investigation traditionally was more of an art than a science. You learned the art and some experience from people who came before you," said Alfred Pisani, an arson expert hired by Dougherty's lawyers. "If you saw a pool-shaped burn pattern on the floor, that meant gasoline might have been used. Now we know it's not true."
One major change came with the recognition of a phenomenon known as flashover. When a fire burns inside a room, it sends smoke and energy toward the ceiling, making the room hotter. When the room reaches a certain temperature, everything combustible in it ignites nearly simultaneously.
The resulting damage can produce signs that were once thought to be indicators of arson, such as burn patterns on the floor that appear to be multiple starting points for a fire. Flashover can occur in a matter of minutes, challenging old beliefs that only fires set with accelerants can burn that fast.