Kwon Hyok makes an unlikely defector. His flat cap and three-piece herringbone suit hangs ridiculously on his thin body. He looks like a cartoon character ready for a game of golf.

The 45-year-old is one of about 4,000 North Koreans who have left their country and now live in South Korea's capital, Seoul. They are generally called "defectors" but most are simply "escapers" who have been lucky enough to make it. Most who embark on the journey from North Korea across the Tumen river to China get stuck on the way. The Chinese are all too willing to send them back, if caught, to North Korea's prisons, and sometimes death.

Kwon Hyok, however, had it easy. There was no arduous two-year trek for him through China, Mongolia and Laos, hiding, pretending and bribing the authorities. He was a North Korean intelligence agent stationed in Beijing and was turned by his South Korean counterparts. He was persuaded to defect. Today, the South Korean taxpayer looks after him and, in return, he advises the government in Seoul on how to deal with the new enemy, the North.

The other North Koreans I met living in Seoul were victims of their old regime. I listened to their dreadful experiences of starvation, cannibalism, torture and murder, and wondered how such brutality could exist.

It was Kwon Hyok who helped to answer my question. He says that in 1993 he was the head of security at Prison Camp 22 in the north-east of the country. It is one of a network of prisons in North Korea modelled on the Soviet Union's Gulag. Most of the prisoners have been charged with no crime. They are there because of the "Heredity Rule".

Kwon Hyok explained: "In North Korea political prisoners are those who say or do something against the dead President Kim Il Sung, or his son Kim Jong Il. But they also include a wide network of next of kin."

Prison camps are filled with relatives of offenders: grandparents, children, brothers, aunts, all arrested for one person's false move and who often have no idea why. Kwon Hyok said: "There is a watchdog system in place between members of five different families. So if I were caught trying to escape, then not just my family members but all of the rest of the four neighbouring families are shot out of collective responsibility." In this way families are forced to spy on each other.

Camp 22, he said, is "surrounded by a 3,300-volt electric fence and, inside the fence, is a 10-metre moat with spikes sticking out to impale anyone attempting escape".

Torture, he said, was routine. "Prisoners were like pigs or dogs. You could kill them without caring whether they lived or died."

He described water torture, hanging torture, the box-room torture. And he described how he ordered public executions in the camp, and not just of those who tried to escape, but of their entire families and the families of their neighbours.

"Once," he said, "I killed 31 people, all members of five families." For the first three years you enjoy torturing people but then it wears off and someone else takes over. But most of the time you do it because you enjoy it."

Kwon Hyok, however, had something else he wanted to tell. He has been in Seoul for almost five years but he has never spoken publicly of what he says he saw in a secret, restricted area of Camp 22.

He said that he witnessed chemical experiments being carried out on political prisoners in specially constructed chambers hidden in the camp. Various different gases, he claimed are being tested there including one he called Vinyla - related to the North Korean artificial fibre Vinalon. He described the chambers - glass rooms within a room - sealed and with a ventilation shaft that pumps gas inside. Above, there is a viewing gallery where, North Korean scientists observe the death throes of their victims.

"The most unforgettable scene I remember was when I watched an entire family being killed. They were put inside the chamber and I saw them all suffocate to death. The last person to die was the youngest son who was crying for his parents and eventually died."

Kwon Hyok said that the youngest victims were children and the eldest were in their sixties. They were selected by others and brought to the chamber where they were stripped naked and given a medical. They had to be checked free of disease before entering.

He drew the layout for me explaining that individual victims stand around the edges of the chamber while families collect in the centre clinging together. "Even though they were dying," he said, "I saw the parents trying to save their children by giving them mouth-to-mouth resuscitation."

"How did you feel when you saw the children die?" I asked. "I had no sympathy at all because I was taught to think that they were all enemies of our country and that all our country's problems were their fault. So I felt they deserved to die."

I listened to his cold and logical testimony remembering the phrase "the banality of evil". His words lacked emotion. He appeared to feel no remorse. He seemed proud that he had earned promotion in the army on the strength of his cold-blooded ruthlessness.

There have been many rumours of human experimentation on political prisoners in North Korea, but never has anyone offered documentary proof. Until now. In Seoul I met Kim Sang Hun, a distinguished human rights activist. He showed me four documents that he told me had recently been snatched illicitly from Camp 22.

They were headed Letter of Transfer, marked Top Secret and dated February 2002. They each bore the name of a male victim; his date and place of birth. The text read: "The above person is transferred from Camp 22 for the purpose of human experimentation with liquid gas for chemical weapons."

The location was named: Vinalon Plant 2.8. There was a North Korean stamp saying Prison Camp 22.

Kim Hang Sun was convinced that this was not a forgery. The paper, the handwriting, the bureaucratic format, the official stamps and the document's provenance all convinced him that it was authentic.

I took one of the documents to a Korean specialist in London who examined it and confirmed that there was nothing to suggest that it was a forgery. I wanted to run a check of my own with Kwon Hyok. Without showing him the Letter of Transfer, I asked him, without prompting him in any way: "How were the victims selected when they went for human experimentation? Was there some bureaucracy, some paperwork?"

"When we escorted them to the site we would receive a Letter of Transfer," he said.

Kim Hang Sun had two explanations for why Kwon Hyok seemed so lacking in emotion. First, they are damaged people, he said, brought up in a perverted value system. But second, he said that many defectors bring stories of chemical weapons experiments in North Korea and they are always surprised at the shocked reaction. In North Korea, they claim, it's common knowledge. They are surprised that we're surprised.