(Reuters) -- Colin Firth has signed on to star as Ron Lax, the private investigator whose detective work helped get three men who had been convicted of murder released from prison in Arkansas, the producers of the Devil's Knot told TheWrap Wednesday.

Lax really is the unsung hero of this story, producer Elizabeth Fowler said.

The West Memphis Three are Damien Echols, Jason Baldwin and Jessie Misskelley, Jr., who were convicted of killing three 8-year-old boys in West Memphis, Ark., in 1993. Their guilt has been questioned ever since, with filmmakers taking up the case in a series of documentaries.

The Devil's Knot will take a dramatized approach to the case, which shot back into headlines in August, when a judge ordered the West Memphis Three freed.

The judge freed the men based in large part on evidence that Lax found. The private investigator -- who worked on the case at no charge for more than a decade because he opposes the death penalty -- found DNA evidence that linked the stepfather of one of the murdered boys to material that was used to bind one of the victims.

The movie, however, is not about how they got out of prison, producer Clark Peterson told TheWrap. It's about how they got in.

The story is told through Lax's eyes and through the eyes of Pam Hobbs, whose son, Steven Branch, was one of the victims. Ultimately, Lax found DNA evidence linking Hobbs' husband to the crime scene.

Lax, originally from Memphis, Tenn., built a successful private investigation business that is one of the largest in the Southeast. When he heard that the state of Arkansas planned to pursue the death penalty against Echols, Baldwin and Misskelley, he offered his services.

But he was reluctant to participate in a movie, according to Fowler.

He's a private investigator, Fowler said. He's not a person who seeks attention or publicity.

She said that she convinced him to participate in the project so we could tell the story with him as our male lead because it was important to get this story told, and a feature film would reach many more people.

Joe Berlinger and Bruce Sinofsky filmed three Paradise Lost documentaries about the case, and Peter Jackson produced West of Memphis, which screened at Sundance.

The Devil's Knot is based on journalist Mara Leveritt's book. Atom Egoyan is directing the movie, which begins shooting this summer.

In addition to Fowler and Peterson, Richard Saperstein and Paul Harris Boardman will produce.

Boardman and Scott Derrickson are writing.