In a report to the Texas Forensic Science Commission released Tuesday, Craig Beyler said the fire investigation in Cameron Todd Willingham's case didn't adhere to the standards of care in place at the time, nor to current standards.
Beyler, chairman of the London-based International Association for Fire Safety Science, said in the report that the opinions of a state fire official in the case were "nothing more than a collection of personal beliefs that have nothing to do with science-based fire investigation."
The commission, created in 2005 to review forensic misconduct allegations, requested the independent analysis after the Innocence Project submitted claims of questionable evidence in the cases of Willingham and another man who was convicted in a similar case but was later released.
Commission Chairman Sam Bassett called Beyler's report "a major step" in the panel's review of both cases.
Before issuing its final report, the commission will seek responses from the State Fire Marshal's Office and other parties, and will interview Beyler in October, Bassett said.
He said he expects the commission to release its report next spring.
Beyler said that in both cases, "The investigators had poor understandings of fire science ... Their methodologies did not comport with the scientific method or the process of elimination."
He said Manuel Vasquez, a deputy state fire marshal in the Willingham case, appeared "wholly without any realistic understanding of fires and how fire injuries are created."
Beyler said witnesses contradicted Vasquez's arson hypothesis and that Vasquez admitted he had not eliminated other possible causes.
Eric Ferrero, spokesman for the Innocence Project, a New York-based organization dedicated to exonerating wrongfully convicted people, said Beyler's findings on the Willingham case "confirms what several experts have found over the last five years after reviewing thousands of pages of evidence."
"Every expert who has looked at this case has determined there was no reason to call it arson," he said.
Willingham, 36, was executed in 2004. He was convicted of setting the fire that killed 2-year-old Amber and 1-year-old twins Karmon and Kameron on Dec. 23, 1991, in their Corsicana home.
He told The Associated Press before his execution that he was innocent. "The most distressing thing is the state of Texas will kill an innocent man and doesn't care they're making a mistake," he said.
Willingham's cousin, Patricia Cox, of Ardmore, Okla., said she has never doubted her cousin's innocence. Family members tried for years to free him.
"I would definitely like the state of Texas to take responsibility and admit in fact they wrongfully executed Todd Willingham," she said. "Is that going to happen? Probably not. I'm not optimistic."
Willingham's stepmother, Eugenia Willingham, called the report another step in the "long, drawn-out process" of clearing his name.
"He lived 12 years on death row," she said. "He went through hell, I'm telling you. It was probably worse than hell."
She said her husband died in 2005, the year after his son's execution, of prostate cancer and "a broken heart."
Vasquez investigated the case with Douglas Fogg, the assistant Corsicana fire chief. The report said they cited burn patterns on the floor of the children's room, hallway and porch, indicating an accelerant spill. Beyler said those determinations have no basis in modern fire science.
Ben Gonzalez, a spokesman for the Texas Department of Insurance, of which the State Fire Marshal's Office is a part, said he had no comment on the report, saying officials there had not yet seen it. He said Vasquez died in 1994.
A call to a Douglas Fogg in Corsicana was not immediately returned Wednesday.
In the other case cited in the report, Ernest Ray Willis was convicted in 1987 in a fatal house fire in Iraan, but was freed after 17 years on death row when a federal judge ruled that authorities concealed evidence and needlessly drugged him during his trial.