Why? The insistent question that demands an answer but gets none. The who and the how are painfully well documented. Anti-semitism, racial superiority, tribal hatred or ethnic cleansing - however depraved and perverted at least they provide some explanation for the events of the Holocaust, Srebrenica and Rwanda. But here there seems nothing - only killing for killing's sake. Death as a way of life.

We have travelled from Vietnam into Cambodia. Five hours by fast boat up the Mekong river to Phnom Penh. Now we are visiting the Toul Sleng Genocide Museum. Toul Sleng was the notorious S-21 Khmer Rouge prison. Set up in a former school, more than 13,000 people were tortured and murdered here or in the nearby Choeung Ek 'killing fields'. Toul Sleng was reserved for the educated and the elite: doctors, teachers, army officers, even government ministers and party cadres. Wives (the photo is of the Minister for Information's wife and baby), husbands and children would be brought here as well; based on the premise that, as an old Cambodian proverb has it, to kill the grass you have to pull up the roots.

The prison is right in the centre of Phnom Penh but once you walk through the gates the noise and bustle of the city seems to fade away. Most of the visitors are Westerners, young people, travellers - though demonstrating little of the lightheartedness you normally associate with them. In marked contrast to Ba Chuc yesterday there are no Cambodians to be seen apart from the guides and staff.

One building was reserved for the most important prisoners - 1 prisoner to a classroom. They still contain the iron bedsteads to which prisoners were shackled. Some have other items in - the jerry can that served as a toilet, the leg irons used to chain the prisoners to their beds. On the wall of each of the cells is a grisly photograph. It shows the corpse of the room's final occupant - still chained to the bed when the Vietnamese arrived in 1979. In most it is barely possible to make out that this is a human being.

One of the other buildings has been preserved as it was found. Barbed wire and mesh cover the entrances and windows. On the ground floor, each former classroom has been subdivided into 8 tiny cells, barely big enough to lie down in, by crude brick walls. On the second floor, the old wooden benches have been used to create equally small cells for women prisoners. These at least have a door with a small window in for the guards to check the prisoners were still alive. A dead prisoner might also lead to a dead guard.

For what the Khmer Rouge wanted from here were names. Firstly, force prisoners to confess that they were spies for the CIA, KGB or Vietnamese. Then to name their confederates so they too could be arrested and brought here. And name more names in a macabre roll-call of death.

To extract the confessions and the names prisoners were tortured. In another building, the methods used are detailed in a series of paintings done by one of the few survivors. Electric shock treatment, pulling finger nails out and, to show they were bang up to date, water-boarding, were just some of the techniques used. In a cabinet on the wall are some of the instruments they used. The Khmer Rouge deliberately used everyday implements and tools for their torture and killing - even making macabre use of the school gymnasium equipment - because bullets were too expensive to waste on killing enemies of the people.

On display boards throughout the prison are photographs. Most are of the prisoners taken as they were brought in (they even built a special chair to simplify the process). Some faces display defiance, others despair but most just seem to show indifference. Other photographs are of the guards. The Khmer Rouge deliberately used young men and women - mostly under 20. Sometimes it is difficult to tell guards from prisoners (though not, of course, when it comes to the photographs taken of people after they had been tortured). And this highlights one of the painful truths of all genocides. They may be instigated and ordered by crazed ideologues, but ordinary people carry them out.

Coming out we meet one of only seven survivors from Toul Sleng. Chum Manh was imprisoned here for over 2 months. He managed to survive because he was able to fix the prison's one and only typewriter. Now 81 he works to raise money to support other victims of the Khmer Rouge. He has also given testimony at the Cambodian genocide tribunal that is, painfully slowly, trying the most senior surviving members of the Khmer Rouge. Our guide asks us if we want to ask him anything. I ask how long was he imprisoned and how did he survive but then the words dry up. What can I say to a man who has endured so much?

Francois Bizot is a French ethnologist who lived in Cambodia who was captured by the Khmer Rouge in 1971 when they were still guerrillas fighting the Cambodia Government and based in the forests in the north. He tells how after his capture he made friends and played with a four-year-old Cambodian girl in the camp where he was being held. After a short while, she was taken away for indoctrination. Shortly afterward, she returned but no longer to play. Every night she would test his bonds and if she could slip her fingers inside them, she would tell the guards to tighten them even further. Which makes me wonder if I haven't been asking the wrong question. Perhaps the question that demands an answer is not why, but why not?