"There were a few critics that were prepared to engage with the film and to take it seriously, but for the most part there was this kind of dismissal because they already knew the story," he says.

"There is a kind of elitism where they don't understand that this is a very powerful story that deserves re-investigation, and that there are a lot of people who might go and see a film with Colin Firth and Reese Witherspoon who would not see a documentary. So when they say, 'Watch the documentaries', I don't think that's really responsible."

For Egoyan, the tale of the West Memphis Three isn't just another murder story, but "one of the most extraordinary pieces of mythology in contemporary American culture".

Three teenagers - Damien Echols, Jessie Misskelley Jnr and Jason Baldwin - were convicted of the killings. They were released after 18 years in jail - a rare deal negotiated with the state of Arkansas set them free, though they remain convicted felons - and to this day, no one knows for sure what happened on May 5, 1993.

Egoyan's film feels haunted by the young victims - Stevie Branch, Christopher Byers and Michael Moore - whom, he agrees, have sometimes been squeezed out of the frame by the tight focus put on the plight of the supposed killers. The boys' fate, though, is the mysterious heart of the story: they went out to play in the woods together, and no one - except for their murderer(s) - saw them alive again.

"Something happened that was so unimaginable," says Egoyan. "These three boys went into this forest and the next day their naked, mutilated bodies are found, tied with their shoelaces, submerged in the water, with absolutely no evidence. No DNA. No blood. No footprints. No branches moved. It's eerie, and it is supernatural. It is the stuff of mythology. You couldn't create that."

The subsequent police investigation, arrests and trials (Misskelley was tried separately because he had confessed) took place amid a "Satanic panic", making Echols - a young Goth interested in witchcraft and heavy metal music, and, significantly, the only one of the three suspects to be sentenced to death - a particularly easy target for a police force and community desperate for answers.

"He was an outsider and didn't understand - or he wasn't coached properly - that one of his roles playing a defendant is to feel innocent," says Egoyan. "He never did. It's quite odd. He maintained his outsider role and actually seemed to revel in it."

In the film, Ron Lax (Firth), a private investigator working for the defence, and Pam Hobbs (Witherspoon), the mother of Stevie Branch, both initially believe that all of the suspects are guilty. However, their certainty becomes eroded by doubt, and they are left despairingly adrift in a sea of confusion. "The real question I wanted to deal with in this film is the notion of powerlessness," Egoyan says.

"It's a very different point of view to a documentary. A documentary is about pointing a finger and saying, 'Go after this person'. I'm not interested in that. I'm interested in this idea of bringing the viewer into a place where you want something to resolve, but it doesn't. And that is ultimately the . The more you try to undo it, the tighter it gets."

Though he didn't write the screenplay, and is his first feature based on a true story, it fits neatly into an oeuvre that frequently includes characters seeking the truth, or who construct false realities to cope with their lives, or are wrestling with questions of guilt and responsibility.

Asked why these themes and ideas keep recurring in his work, the filmmaker says that it comes partly from growing up as the son of Armenians, 1.5 million of whom were killed by the Ottoman government in the Armenian genocide of 1915, although Turkey still refuses to recognise the event.

"You're dealing with the fact that there's this cataclysmic historic event which you're taught is an absolute reality, but which is denied," says Egoyan, who explored the genocide and its legacy in his controversial 2002 film, . "So you're often in these bizarre conversations and that becomes a part of your formation - I mean to this day - I suppose. So there is that political aspect of it, which is just part of one's upbringing."

On a more personal level, he says he had a "strange experience" where the "young woman I was completely obsessed with, for like five years, during my teen years, was being abused by her father at the same time. I understood that there was something creepy and unusual about it, but she was in denial and, certainly at that time, no one was talking about those issues," the filmmaker says.

"So I think the combination of those two things in my life - the one which is kind of a grand, familial communal tale and this personal one - have had a very strong effect on me," he surmises.

Egoyan isn't optimistic about the real killers ever being brought to book. Pam Hobbs once suggested that Terry Hobbs, her ex-husband and Stevie's stepfather - a man with a history of violence and a strand of whose hair was found in the shoelace binding Michael Moore - might have been involved in the murders. She was on set almost every day and Egoyan says that they had what were often "odd conversations".

"The film doesn't really address this but [Terry] is still in her life. And it's the oddest relationship," the director says.

"He is a dark, malevolent man. But just because he abused his daughter, just because he had violent outbursts, just because he had this dark personality, it doesn't mean he could have done it.

"This is the thing about the crime. The more you look into it, the more impossible it is."

Devil's Knot