The Khmer Rouge killed 1.7 million of its own people through starvation and torture in a bid to create a peasant society free of class structures and foreign influence. Nhem En defends his participation by saying he was just a cog in that killing machine.

'The blindfold was taken off and I was not allowed to talk to the prisoner,' he said. 'The prisoner usually asked: 'Why am I here?''

Nhem En ensured the prisoners did not blink or tilt their heads in the photographs. Women, men and children were forced to stand rigidly with eyes looking straight ahead as he took their mug shots.

The photo would later be pinned to a prisoner's confession, which was extracted through torture.

It is not known exactly how many people passed through the gates at Tuol Sleng, but the figure is accepted as being greater than 14,000 and fewer than 20,000. Tuol Sleng had been a high school in Cambodia's capital Phnom Penh. But under Pol Pot the complex's role changed completely, and in the years 1975-79 anyone detained at Tuol Sleng was suspected of being a CIA agent or traitor.

'I knew they would die afterwards,' Nhem En said dispassionately. He estimates that he photographed half of the victims of Tuol Sleng.

Nhem En also photographed meetings of the Khmer Rouge leaders, and delegations visiting from North Korea and China. Now aged 46, he is seeking forgiveness from his compatriots. 'I would like to offer my apology and say sorry to all the victims of the genocide. I hope all the spirits of the victims are able to rest in peace.'

Cambodians are superstitious and believe that if a person dies violently their spirit will not find peace but instead continue to wander the earth.

Nhem En's apology is remarkable, but then again it isn't. It is remarkable because none of the ageing top leaders of the Khmer Rouge, who are now mostly in their 70s and 80s, have come forward to admit guilt for the regime's brutal crimes. Pol Pot died in 1998 and Chhit Choeun, alias Ta Mok, the former commander in chief of the Khmer Rouge army, died last year.

The other members of the leadership - Khieu Samphan, the nominal head of state during the Khmer Rouge, Nuon Chea, Pol Pot's deputy, and Ieng Sary, the former foreign minister - remain free in Cambodia. It is widely expected that they will be indicted for the Khmer Rouge trial this year.

The other reason Nhem En's contrition is significant is that it's not normal in Cambodian culture to say sorry.

'For a Cambodian to apologise is a very big thing - it's a loss of face,' said Youk Chhang, who founded the Documentation Centre of Cambodia (DCCAM) to record and preserve the history of the Khmer Rouge for future generations.

Much of DCCAM's material - 800,000 documents, 30,000 photos, 200 documentary films, 189 identified prisons and a record of 20,000 mass graves - is now evidence against the former Khmer Rouge leaders.

'I admire [Nhem En's] courage of doing something, if not for all, for himself,' said Youk Chhang. 'If his apology can make a difference among even 10 people, then it's worth it.'

But all the goodwill Nhem En might have generated with his apology was lost with his announcement that he intended to profit from the photos he took at Tuol Sleng.

Nhem En is a short, thickset, dark-skinned man with gold-rimmed glasses. He has two pens in the pocket of his white business shirt. Nhem En left Tuol Sleng with only his camera, film and two pens when the Vietnamese took control of Cambodia in January, 1979, and ousted the Khmer Rouge. He is now deputy governor of the town of Anlong Veng, a former Khmer Rouge stronghold in the country's northwest, and is also studying for his high-school diploma. He wants to set up in a museum in Anlong Veng to exhibit his photographs.

'I cannot say right now if I will sell the photographs or not,' he said. 'I will have to get permission from the government and if they say I can sell them then of course I will sell them.' When Nhem was interviewed by Nic Dunlop for his book The Lost Executioner, which is about Kang Khek Leu, better known as Duch, the former schoolteacher who was in charge of Tuol Sleng, Nhem En demanded US$300 for the interview. Dunlop bargained him down to US$55.

Nhem En talks fast and as he becomes emotional his voice turns shrill. He changes his mind about the sale of the photos and tells me instead that he will seek the advice of fellow journalists and photographers as to whether it is the right or wrong thing to do.

Has he no moral compass of his own? He again changes his answer: he won't sell the photographs of the victims at Tuol Sleng, only those of the top Khmer Rouge leaders.

His planned museum and selling of photos of the Khmer Rouge's gruesome past have caused mixed feelings in Cambodia, a country drenched in suffering and frustrated by its long wait for justice.

Nhem En says the reason he's apologising and establishing the museum is to help with 'national reconciliation'.

It's hard to heal a nation like Cambodia. Like a shattered glass it's hard to put back together as it once was. Even if you succeed, it's still not quite the same.

Nhem En could have donated his collection of photographs to Youk Chhang of DCCAM, which has a policy of not paying for any of its material. Instead, DCCAM will scan the photos and help mount them at Nhem En's museums, after the centre has discussed with the photographer how he will explain the message behind each picture.

The Khmer Rouge remained active in parts of Cambodia during the Vietnamese occupation that lasted until 1989, and continued fighting until 1999. Nhem En stayed as a Khmer Rouge photographer, taking photos of its battalions, and later travelled with Pol Pot, who avoided arrest, to Thailand, North Korea and China. Nhem En didn't defect from the Khmer Rouge until 1995 - the year he finally returned to Tuol Sleng.

Tuol Sleng was converted into a museum by the Vietnamese. On the walls are many of Nhem's photographs. The black and white mug shots of the victims have been lined up together and stare at you like the living dead. They are the evidence of the Khmer Rouge's crimes, and those faces are still waiting for justice.

Tuol Sleng comprises five buildings that surround a grass courtyard. Inside the buildings, visitors will find rusting iron bed frames where prisoners were tied and tortured; leg irons for prisoners; instruments of torture; and paintings that depict infants being thrown in the air and speared by soldiers holding bayonets.

Tourists wander Tuol Sleng alongside Cambodians who still go to see if a member of their family is pictured on the walls. Still, there are many young Cambodians, born after the Khmer Rouge era, who do not believe that the atrocities occurred.

Youk Chhang received approval recently from Cambodia's Ministry of Education to provide about 500,000 copies of a book to high schools that will teach of the crimes of the Khmer Rouge. It will explain how many Cambodians died and include individual victims' stories.

Joseph Mussomeli, US ambassador to Cambodia, sees the photographs as an important history lesson. 'We all know from recent surveys that many young Cambodians do not even believe the genocide occurred, that their parents must have exaggerated the horror, but these photos remind everyone of the true enormity of that crime,' he said.

Nhem En used to print photos of the victims of Tuol Sleng in a house only a few doors away from the prison. 'In 1995, when I first returned to Tuol Sleng, I was very moved,' he said.

It is highly likely he will be called as a witness at the Khmer Rough trial this year to describe the inner workings of the regime. He will not be prosecuted, as he was a lower ranking cadre. He already has a prepared line for when he takes the witness stand: 'Pol Pot is definitely not a hero, he's a criminal like Saddam Hussein.'

No doubt Nhem En will have to defend himself against questions about why he did nothing to help save his compatriots. His matter-of-fact reply is that he feared for his own life.