Filmmakers Joe Bailey Jr. and Steve Mims, directors of ‘Incendiary.’ (Chris Bailey Photography )

In recent years, more than 250 Americans convicted of serious crimes have been freed thanks to sophisticated analysis of DNA and other forensic evidence. “Incendiary: The Willingham Case,” a documentary that opens in Washington on Friday, is not about a case like that.

Cameron Todd Willingham, the man at the story’s center, was convicted of a horrible crime: murdering his three young daughters by setting the family house on fire. But he can’t be released from prison because he was executed in 2004. And he can’t exactly be exonerated, his champions say, because there was no crime; the fire that killed the three girls was accidental, they say.

“What fascinated us most,” says “Incendiary” co-director Joe Bailey Jr., “was that you had people who were close to the Willingham case who were adamant that he was guilty, with no reservations. And you had a growing chorus from the scientific community that there was no evidence of arson whatsoever.”

Bailey, 29, was in elementary school in Houston in 1991 when the girls died in a town in northeast Texas. Co-director Steve Mims, 52, remembers hearing about the case, but only in passing. Then in September 2009, the New Yorker published David Grann’s lengthy account of Willingham’s alleged arson and the lingering controversy. Bailey, a law school graduate who is not a lawyer, was taking a film course taught by Mims.

“He said, ‘Somebody should make a movie about this,’ ” Mims recalls. “And I wrote him back, ‘That’s a lot of work.’ But then I thought, ‘Well, let’s just go ahead and do it.’ ” Neither man had ever attempted a full-length documentary, and the longtime Austin residents were involved in filmmaking mostly to document the local music scene.

While Grann’s article holds the arson analysis until its final section, “Incendiary” opens with it. “We emphasized the science mostly because we didn’t see it done elsewhere,” Bailey says. “We wanted to give the audience that knowledge early in the film, so that they would have a point of reference for the case.

“The scientists get short shrift, usually,” he continues. “They get their 30 seconds to say, ‘There’s no evidence of arson here.’ And then the media moves on.”

Gerald Hurst and John Lentini, the two fire experts who reject the arson verdict, are not directly challenged in the film. Manuel Vasquez, one of the fire marshals who claimed the fire was set, has since died. Most of Willingham’s other official antagonists refused to be interviewed. These include Texas Gov. Rick Perry, who denied a stay of execution in the case.

“Perry and others characterized Willingham as a monster,” Mims says. “It makes people emotional about something that really, at its core, is not about that at all. It’s about, is there any evidence at all of what he was convicted of?”

Perry, of course, is now a candidate for the Republican presidential nomination. “Before he announced,” Mims says, “people would look at the movie and say, ‘Well, this is interesting, but it’s sort of just a Texas story.’ It’s become more than that now.”

Among the movie’s other supporting characters is James Grigson, a Texas forensic psychiatrist who has testified in so many capital cases that he has become known as “Dr. Death.” Grigson’s role was central to “The Thin Blue Line,” Errol Morris’s documentary about a man who was convicted of murder but ultimately released.

“We were fascinated by that connection,” Bailey says. “An early cut of the film featured more of Grigson. We cut it back because we didn’t want to invite the comparison to ‘The Thin Blue Line.’

“It’s a small world, the Texas legal system,” he adds, chuckling.

The documentary also includes footage of John Bradley, Perry’s choice to head the Texas Forensic Science Commission, which was charged with reevaluating the Willingham evidence. Bradley had the camera-wielding Mims ejected from one commission meeting but was forced to readmit him after Bailey pulled up the Texas Open Meetings Act on his smartphone.

Mims calls Bradley “an amazing character in the film. He behaves in a way that you would never believe people would behave on camera. Nobody can say for sure, but he apparently did the job he was put in there for. He successfully stretched the case out, beyond the governor’s reelection” in 2010.

Curiously, the movie’s principal anti-Willingham voice is the executed man’s defense attorney, David Martin. He insists loudly that his client was guilty. “That’s such a bizarre thing for an audience,” Bailey says. “I think that’s part of the appeal of the film. That makes it so strange and interesting.”

Neither filmmaker opposed the death penalty when they began “Incendiary,” and they still don’t. “I definitely can see cases in which it seems appropriate,” Bailey says. “But the knowledge that a possibly innocent person was executed is extremely troubling to me. It’s one of the reasons I was so interested in this case.”

“There’s such regularity in Texas about executions, and to me it seems to be a rubber-stamp process,” Mims says. “People who just read the newspaper, read about somebody being executed and the person had 12 appeals and it went all the way to the Supreme Court — they think, ‘The system must work.’ This is a great example of how you can read that same story and realize, ‘Wow, it doesn’t work at all.’ ”

Jenkins is a freelance writer.

Incendiary: The Willingham Case

(104 minutes) is not rated. Opening Sept. 30 at Landmark’s E Street Cinema.