By the time firefighters put out the flames that had engulfed Daniel Dougherty's brick row house, it was too late.
Neighbors said Dougherty, who said he'd fallen asleep on the couch, ran out of the house and tried to put out the fire with a neighbor's hose. A police officer later said that Dougherty, standing shirtless outside his North Philadelphia house in the near dawn Aug. 24, 1985, said, "My name is mud. I should die for what I did."
By then, fire and smoke had moved into the upstairs bedroom and killed Dougherty's two sons, John, 3, and Danny, 4.
Sifting through the wreckage and examining the burn patterns left by the flames, John Quinn, an assistant fire marshal, concluded the fire was a case of arson.
At Dougherty's trial, Quinn said the fire started nearly simultaneously in three separate places -- on the sofa and love seat and beneath the dining room table. The multiple points of origin, he said, were a sure sign the fire was intentional.
No one questioned his conclusion. It took a few hours for a jury to convict Dougherty of murdering his two sons and he was sentenced to death.
But several leading arson experts now say there was no scientific evidence of arson that night. Quinn, these experts say, relied on long-held but now discredited beliefs about how fires behave and the marks they leave behind.
"He was so far off base it's unbelievable that no one challenged his expertise or methodology," said John Lentini, a nationally known fire investigator who is a defense expert in Dougherty's motion to vacate his conviction. "Anyone paying attention for the last 20 years would have known that what he was saying was totally off the wall."
Those outdated techniques once commonly used by fire investigators, Lentini and others warn, may have landed dozens of innocent people in prison for fires they did not start. Dougherty's case is one of what is expected to be a string of legal challenges to arson cases from the 1970s, '80s and early '90s.
"I didn't set any fire. I'm an innocent man on death row and I have a child out there who needs my help," Dougherty said in a recent telephone interview from Pennsylvania's death row.
A Philadelphia judge is now considering Dougherty's motion to vacate his conviction and is expected to rule in early October. The District Attorney's Office declined to comment, but in court papers it argued that Quinn's opinion was valid and that there was ample evidence that Dougherty intentionally had killed his children. Quinn, now retired, declined to comment.
"Those opinions [of Dougherty's experts] are nothing more than Monday morning quarterbacking and would not have affected the outcome of defendant's trial," prosecutors wrote.
Though it's unclear how many convictions may have been based on what these experts say is outdated science, experts consulted by ABCNews.com put the number at anywhere from a few dozen to a few hundred. Researchers are only now beginning an effort to catalogue a comprehensive list of questionable cases.
"I'd have to say 10 [percent] to 20 percent of the cases decided in the 1980s and early 1990s were probably wrong, or could have been wrong," said John DeHaan, a former arson investigator with the California Department of Justice.