More than 30 years after his two young sons died in a house fire, and nearly 17 years after he was sentenced to death for setting it, Daniel Dougherty will get a new chance to prove what he has always insisted:

He didn't murder his children, he loved them and tried to save them the night their Oxford Circle house burned in 1985.

Dougherty, 56, has been granted a new trial in Philadelphia Common Pleas Court after an appellate court found that his lawyer's failures so skewed the original proceeding that "no reliable adjudication of guilt or innocence took place."

His retrial is scheduled to start Monday.

Dougherty's new lawyers are expected to present expert testimony to show that the fire science used to convict him was bunk, that what sounded like proof of arson actually proved nothing.

A spokesman for the Philadelphia District Attorney's Office declined to comment on the upcoming trial. Prosecutors vigorously opposed a retrial, contending in court that ample evidence supported the original jury finding of guilty.

In 2000, the jury took only three hours to convict Dougherty of murdering 4-year-old Daniel Jr. and 3-year-old John, who died of smoke inhalation.

"The important issue here is the science," said Marissa Boyers Bluestine, legal director of the Pennsylvania Innocence Project, which works to exonerate people convicted of crimes they did not commit, and which has assisted Dougherty. "I'm not convinced [prosecutors] can meet the standard that a crime occurred."

Nationally known investigator John Lentini, of Scientific Fire Analysis L.L.C. in Florida, said the cause of the blaze should have been ruled "undetermined."

The extensive damage to the brick rowhouse, he wrote in a report for the defense, made it impossible to determine where or how the fire started. What's certain, he said, is that the evidence did not show three separate points of origin, the basis for the prosecution's conclusion that the fire was arson.

Dougherty has always said he was asleep on the living-room couch, his children in their second-floor bedroom, when he awoke to see the curtains on fire. He ran outside, then tried to reach the boys before being forced back by heavy smoke and flames.

Experts who study arson convictions compare Dougherty's case to that of Cameron Todd Willingham, executed in Texas in 2004 for killing his three young daughters by setting the family home ablaze. Five years later, an expert hired by the Texas Forensic Science Commission called the arson finding into question, and said better understanding of fires could have freed Willingham.

Around the country, prison inmates have challenged convictions they say are based on old, disproven science. For instance, in 2014, after 24 years in prison, Han Tak Lee was exonerated and freed, having been convicted of killing his daughter in a 1989 Pocono Mountains cabin fire.

Dougherty's appellate lawyer, David Fryman of Ballard Spahr, declined to comment on the pending trial.

Dougherty's death sentence was vacated in 2012, becoming a life sentence, after prosecutors agreed with the defense on the ineffectiveness of his trial lawyer, Thomas Ciccone, now deceased. Dougherty was moved from state prison to the Curran-Fromhold Correctional Facility in Northeast Philadelphia to await retrial.

Dougherty was a suspect from the start, but was not arrested until 14 years after the fire, in 1999. That's when his estranged second wife, Adrienne Sussman, then battling Dougherty for custody of their child, called police and told them he had confessed to her. She never testified at trial.

The prosecution said Dougherty set the fire to harm two women - his girlfriend, Kathleen Schuler, with whom he lived on Carver Street, and the mother of the boys, Kathleen Dippel, from whom he was separated. He destroyed the house of one and the children of the other, prosecutors said.

Dippel, who was not called to testify, said after the trial that Dougherty loved their sons and that she believed he was innocent.

Key prosecution testimony came from John Quinn, who 15 years earlier was an assistant city fire marshal. He said the fire started in three places. Dougherty's descriptions of his rescue efforts were not believable, he said, because his body showed no exposure to flames or smoke.

Dougherty's lawyer, Ciccone, never challenged that testimony by having an expert address the scientific advances between 1985 and 2000. If the jury had heard and believed such an authority, the Superior Court said in ordering a new trial, it "would have had reasonable doubt about [Dougherty's] guilt."

Now a new court will decide.

"Trials are always risky ventures, for the prosecution or defense," said Bluestine, of the Innocence Project. "But from the vantage point of the convicted innocent, being granted a new trial means a chance to finally have a fair trial and a shot at proving one's innocence to the world."

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@JeffGammage