Errol Morris had a simple starting point for his latest documentary Standard Operating Procedure. Intrigued by the infamous snapshots taken by American soldiers at Abu Ghraib prison depicting Iraqi detainees who had been forced to form naked pyramids and adopt other sexually degrading poses, the acclaimed US director of The Thin Blue Line and The Fog Of War wanted to dig deeper.

Setting out to examine in forensic detail the context in which each of the notorious photos were taken, Morris explores whether they constituted evidence of systematic abuse by the US military or were merely the efforts of a few aberrant 'bad apples' among the soldiers.

'It's an inquiry into how those photographs both inform and mislead,' says Morris. 'The whole phenomenon of Abu Ghraib has been mischaracterised. The photographs are both a cover-up and an expose because in one sense they force you to examine what was being done there but they did not cause people to look further, as if somehow that's all that there was.'

The film-maker, who won an Oscar in 2003 for The Fog War, convinced five of the seven indicted military policemen (the other two are still in prison) to take part in the film. The highest-ranking military official he interviewed was Brigadier General Janis Karpinski, who as head of prisons in Iraq was relieved of her command following the scandal, despite insisting she had no knowledge of what was happening. But, as Morris says, the film is about the people 'at the bottom of the pyramid' who were demonised for their actions, letting those higher up the military hierarchy - who shaped Abu Ghraib's abusive culture - off the hook.

'Someone like Lynndie England, who was 20 when all this happened, became emblematic of all of it - as if she herself was the stain of the war,' argues Morris. 'We like to think that people who are in this kind of a prison setting, where abuses are occurring on a routine basis, are all automatons who have stopped thinking about moral or ethical questions. That's clearly not true.'

With impressively rendered graphics and animation, meticulously staged re-enactments and a broody score by Danny Elfman, Standard Operating Procedure looks primed to take advantage of the burgeoning market for upscale, slickly produced documentaries that have the potential to reach beyond the arthouse market and make a dent in multiplexes.

The film, which won the Silver Bear Jury Grand Prize at February's Berlinale, was co-produced and co-financed by Participant Productions and Sony Pictures Classics (SPC) with Morris pegging the budget at 'around $5m'.

And the fiery film-maker says he is not finished with this story: having scrutinised Abu Ghraib's scapegoats, he wants to dig deeper. 'Frankly, I would like to see the people who were responsible for this punished. Many people involved in Abu Ghraib have been censured. But the people who are responsible for these policies have emerged unscathed. They pin medals on each other's chests and congratulate themselves.'

SPC is releasing the film in the US on April 25.