A mother looks out of the window as her son goes out on his bike to play with two fellow eight-year-olds.

They are never seen alive again.

It’s May 5, 1993 in West Memphis, Arkansas, where the bodies of Stevie Branch, Christopher Byers and Michael Moore would be found the following day in a muddy creek at Robin Hood Hills.

So begins this reconstruction of this real life crime that shocked America.

West Memphis’ deeply religious community sought justice and the finger of suspicion pointed at 18-year-old Damien Echols, a heavy metal fanatic with an interest in white witchcraft, and his two friends, 17-year-old Jessie Misskelley Jr and 16-year-old Jason Baldwin.

After hours of police interrogation, Misskelley Jr confessed to the murders and the three teenagers stood trial. Baldwin and Misskelley Jr were sentenced to life and Echols to death by lethal injection.

More than 18 years after they entered prison, the men were released on a special plea deal after DNA evidence cast doubt on the convictions

Reese Witherspoon is Stevie’s haunted mother Pam and Colin Firth is a bearded private investigator called Ron Lax, hired by a defence team to cast doubt on the guilt of three young suspects.

On the face of it, the presence of two best actor Oscar winners should have made this a blockbuster thriller.

But knowing it is released when the World Cup is underway tells you that it must be flawed.

Had there not already been several documentaries about this case and had this been a work of fiction, Canadian director Atom Egoyan (The Sweet Hereafter) could have added any number of different spices to play with the audience’s emotions beyond showing us some rather gruesome still images of the dead children.

He would surely have had Witherspoon and Firth ramping up their performances by frequently appearing together.

Instead, they are mostly kept apart in a film that has too many other strands.

It doesn’t quite gel as either a proper whodunnit, an insight into a killer’s mind, a debate about lyrics in heavy metal music or a gripping legal case.

Yet Devil’s Knot is well made with solid performances and does feel relevant in many ways.

And, coincidentally in a year when it will soon be 50 years since the last executions in Britain, Firth’s character is determined to try to make sure that the deaths of three innocents are not followed up with three more simply for vengeance.