A sea of faces filled the boulevards of Phnom Penh. Pandemonium seized the city once known as the Paris of south-east Asia, as two million bewildered residents decamped to the countryside. My family drifted along with the ebb and flow. The old and the infirm tried desperately to fight off the heat, the deluge of bodies and carts, and exhaustion. Some 20,000 Cambodians died during the mass exodus.

Within three days, Phnom Penh became a ghost town. Similar migration occurred elsewhere. No location was exempt. Thereafter, for almost four years, all the towns and cities across Cambodia stood vacant, as their former residents slaved away in labour camps in the countryside.

The journey weighed heavily on my father, who was recovering from his war wounds. He had been hospitalised since March, and it was only at the request of my mother that he had been released a few days before the exodus. With the increase in bombing attacks on Phnom Penh, she wanted all her family together.

Several days later, we arrived at Wat Champa, an ornately decorated temple complex where the city limits of Phnom Penh end and Kandal province begins. Soon, announcements requesting the return of all former civil servants and military personnel to Phnom Penh blared through loudspeakers; their assistance was needed. My father heeded the call, partly out of curiosity, and partly to stock up on food.

"Darling, don't go," beseeched my mother. "You're not fully recovered."

"Please, don't go," I repeated.

"Papa has to go," he said smiling at me. "But I will come back."

Not until many months later did it dawn on us that the Khmer Rouge had lured former Lon Nol soldiers and civil servants - the enemies of the new regime - to their deaths. The Khmer Rouge executed many of these men immediately. They transported them west in military convoy trucks and disposed of them at Pich Nil. Their remains testify to unspeakable barbarity. The other men were held for a time in Tuol Sleng prison before they were executed. Less than a handful of prisoners are known to have survived. In the name of eliminating the bourgeoisie, the Khmer Rouge killed off the entire educated class in Cambodia, and countless others besides.

For us, many weeks passed. The soldiers stopped giving us rice. Instead, they gave us samlay, grain of cotton, an inedible diet that quickened the deaths of first the children and the elderly, then the rest. Once healthy, strong bodies transformed into ghost-like figures stripped of dignity and grace, no longer recognisable as human beings. Death surrounded us. A child immediately to our right starved to death. A woman in front of us died in her sleep. Every family, save ours, experienced the death of at least one or two members during the stay in Wat Champa.

Rather than face a slow death, we escaped one night by crossing a river in a canoe. After walking 90 miles in the monsoon season, we finally reached a village where we were met by my father's father. He took us in, and for a while we were safe.

Two years later, however, the village was overrun by Khmer Rouge soldiers retreating from an invasion by Vietnamese troops. My whole family was arrested and imprisoned. Each night the guards inspected the chains on our ankles. On one occasion, in the next room, a prisoner's ankles were not properly chained. The guards beat him. Another night when one prisoner attempted an escape, the authority accused the others in the room of collaboration and killed them all.

A couple of days before the disappearance of my mother, two prisoners from the adjacent room tried to escape. They were captured and shot. The day following the attempted escape, my mother distributed her belongings to other prisoners. She sensed death's propinquity and calmly prepared to meet it. That evening, the guard unchained my brothers' ankles and two guards escorted them from the compound. The two older boys had a premonition of the evil confronting them. Sina leaned towards Mardi and whispered, "Did you see, there were a lot of guards with ropes, guns and shovels outside the prison compound?"

Later that night, two prison guards peeked in our cabin. This, too, was unusual. They had already secured the shackles on the adults' ankles for the night. One of the guards gave a cursory glance across the room, seeing my youngest brother Daravuth curled up against my mother on one side and me on the other; I caught his eye and he quickly left. "Mom, why were those guards carrying ropes?" "My daughter, go back to sleep."

Sometime before morning the guards took her. I was seven years old; Daravuth was four. Little did I know that would be the last time I would see my mother. The light went out. Eternal night. Life is but a breath.

What seemed like several hours later, my older brothers returned to us. The prison was eerily empty. "Here, take your crying siblings with you. You're free to go home," a guard instructed my older brothers. My mother's blood purchased our freedom. We made our way back to our grandfather's village.

They eventually escaped over the border into Thailand. After months in refugee camps, they left for America. In February 2002, Theary Seng returned to Cambodia.

I learned of my meeting with my parents' murderer less than 24 hours before it was to take place. Over dinner, my host - the facilitator of the legal seminar I was to deliver - nonchalantly asked whether I would like to meet Khieu Samphan. My jaw dropped. "Really?"

"Yeah. I know him quite well."

Khieu Samphan, Khmer Rouge head of state, Brother Number Five in Pol Pot's regime. Khieu Samphan, the popular teacher and government minister who shunned corruption and humbly rode rickshaws to work. Khieu Samphan, always smiling in pictures.

I hold Samphan accountable for the deaths of my parents. I hold him accountable for the deaths of my relatives. I hold him accountable for the blood of 1.7 million others. Yet, to this day, he lives freely among his victims, in Pailin, the former regime stronghold in northwestern Cambodia. His closest aide, as I found out later, attended the seminar the next day. He confirmed the meeting during our lunch break.

The hour I spent with Samphan was one of the most surreal events of my life. As our car pulled into the dirt courtyard of a typical Khmer village dwelling, a man walked out of the wooden structure. My heart skipped a beat. I immediately recognised him as someone I knew well, even though we had never met. Of course, the familiarity came from public pictures. I stood face-to-face with evil incarnate, my parents' murderer. But instead of revulsion, a perverse sense of awe initially captured my emotions - for evil was not mad, but charming, gracious and grandfatherly.

He was my height or a bit taller, smooth-skinned, fair and well-built - more stocky than average Cambodian men. I pressed my palms together to greet him in the customary Khmer manner.

There was nothing in the room except for two green, American-style cushioned armchairs and three regular wooden Khmer chairs, separated by a coffee table. We were served hot water, of which I took several sips. I asked for his thoughts and feelings about 1975 - about the deaths of so many people and how it was the first time in world history that a people systematically killed their own people. True to form, he proclaimed his ignorance of the killings. He asked rhetorically whether he looked like a mass murderer, violent and capable of committing the gross, inhuman acts.

Quickly we both understood each other's position, but in this strangely polite conversation, Samphan and I talked past each other, treading carefully so as not to break the fragile moment. I felt no anger towards him. I was a bit teary, but I was surprised at how calm I was.

When it was time to leave, he walked us to the door. I am amazed at his ability to live with himself, at his ability to convince himself of the rightness of his cause to a degree where he is still functioning well.

I believe the Cambodian population at large is resigned to the inevitability that a legitimate trial will not happen. A UN-backed tribunal may take place, but we are fooling ourselves if we think justice or collective closure will be had. I do not believe the tribunal itself will bring about personal healing. That takes place in the quietness of one's soul.

· Daughter of the Killing Fields by Theary C Seng is published by Fusion Press on September 22 at £15.99. To order a copy go to Amazon UK.