There have been documentaries aplenty about the tragic events in West Memphis, Arkansas in 1993, when three small boys were brutally killed, and three local teenagers convicted of their murder. Directors Joe Berlinger and Bruce Sinofsky followed the story as it unfolded, and ended up making three films for HBO over several years. More recently, Amy Berg’s West Of Memphis re-examined the evidence, and took the story further.

The emphasis has always been on the circumstances of the convictions. A careless police investigation and the assumption that the killings were part of a Satanic ritual led to the singling out of three teenagers whose interest in things such as heavy metal music and Wiccan practices were considered proof of their guilt. One was sentenced to death; two others received life. in 2011, all three were set free.

Devil’s Knot is a little different. Canadian filmmaker Atom Egoyan dramatises the story, based on a true crime account. Names remain the same, and actors have been cast for their resemblance to real people. Only with the two leads does there appear to be some degree of poetic licence. Colin Firth plays Ron Lax, an investigator for the defence, while Reese Witherspoon plays Pam Hobbs, the mother of one of the dead boys. Around them, there’s a condensed version of events, from the day of the murders to the convictions of the three. There are a few creative flourishes - moody images of the woods where the boys’ bodies were found, brief moments when a child becomes a narrator - but Egoyan’s version adds little.

The focus on these two contrasting characters feels forced. Lax is shown as a principled man with a distracting backstory (buying Napoleon III furniture at auction, getting a divorce, spending a lot of time in diners) while while Hobbs is depicted as fragile and raw - yet even in the midst of her grief, she begins to have doubts about the guilt of the accused.

Devil’s Knot has no doubt that there was a miscarriage of justice, but chooses not to throw its weight behind an alternative theory, although it suggests several avenues that could have been explored. But it’s not clear what the filmmakers felt was to be gained by dramatising the story, by constructing a slightly bland and constrained work of fiction..

In literary adaptations such as Felicia’s Journey and The Sweet Hereafter, Egoyan has shown that he can depict darkness and explore loss. But here, he feels mired in stereotypes, or dependent on images that documentaries have already created.

It might have been better, perhaps, if he had simply focused on Witherspoon’s character, rather than on the courtroom elements and evidence anomalies; or if he had taken a different, freer, more imaginative approach.