Twice he stopped and twice he fell to his knees, hands clasped in tearful prayer for the terrible crimes committed three decades ago.
Kaing Guek Eav, better known as Comrade Duch, is now 66 and seemingly frail. But 30 years ago he oversaw a Khmer Rouge torture centre whose victims were taken to the notorious Killing Fields, butchered, and buried in mass graves.
Yesterday, when a UN-assisted tribunal took Duch back to those Killing Fields, where up to a third of Cambodia's population was murdered, he broke down. "It was emotional, of course, and very quiet," said Reach Sambath, a spokesman for the tribunal said from the capital, Phnom Penh. "Everyone was very quiet."
Duch is among five senior Khmer Rouge leaders being tried by the so-called extraordinary chambers in the courts of Cambodia for crimes against humanity perpetrated during the group's brutal reign in Cambodia, when up to 1.7 million people were either killed or died from disease and starvation.
During that time, the former maths teacher was head of the Tuol Sleng prison, otherwise known as S-21, through which more than 20,000 people passed on their way to the Killing Fields. Barely a dozen are known to have survived; today, just four are still alive.
The best-known site of mass graves, littered with bones and pieces of ripped clothing, is Choeung Ek, about 10 miles south of Phnom Penh. Duch, a born-again Christian, was taken there in a heavily guarded convoy with up to 80 tribunal staff, judges, lawyers and four witnesses who served as Khmer Rouge guards at Tuol Sleng.
"The four witnesses explained what had happened in front of the accused, Duch," said Mr Sambath, who said he was not permitted to reveal details of the testimony. "He also explained what had happened there as well. Everything was recorded."
After the testimony had been given Duch broke down, first as he passed a tree bearing a sign that said babies' heads were smashed against its trunk, and second, as he made his way back to his car and stopped at a Buddhist stupa that contains the skulls of more than 8,000 of the Khmer Rouge's victims. "He kneeled on the ground and paid his respects and prayed for the souls of those who were killed," said Mr Sambath. "He cried ... When he got to the car he also paid his respects to the skulls behind the glass. He also knelt on the ground and prayed."
Almost 30 years after the Khmer Rouge were driven from power by invading Vietnamese forces, Cambodia remains haunted by the genocide. Many in the country believe the tribunal is the best opportunity for justice to be brought to the handful of senior regime leaders still known to be alive. Pol Pot, or Brother Number One, the regime's chief protagonist, died in 1998. "This re-enactment is important evidence for the trial," Chea Thoy, who lost 13 relatives including her husband, told the Agence France-Presse. "It will also preserve what happened so that it will not be lost. We can keep it for the young people."
The Khmer Rouge surged to power in 1975, riding on a wave of anti-government feeling that some believe was boosted by the secret US bombing campaign, part of the spillover from the war in Vietnam. Merging a mixture of nationalism with Vietnamese, Chinese and French communist ideologies, the Khmer Rouge established a ruthless agrarian revolution purportedly based on the peasants, which involved the execution and suppression of the educated and urban classes.
Duch had been head of a regional college before the Khmer Rouge. When the regime was forced from power he disappeared, but was eventually discovered by a journalist in 1999 working as a volunteer for the charity, World Vision, in the north of the country.
In an interview with The Independent before he was taken into custody, Duch said he had little alternative but to do what he did at Tuol Sleng. "I, and everyone else who worked in that place, knew that anyone who entered had to be psychologically demolished, eliminated by steady work, given no way out.
"No answer could avoid death. Nobody who came to us had any chance of saving himself. All the prisoners had to be eliminated. We saw enemies eveywhere. If I had tried to flee, they were holding my family hostage, and my family would have suffered the same fate as the other prisoners in Tuol Sleng. If I had fled or rebelled it would not have helped anyone."
The trial of Duch and his fellow accused is scheduled to begin in July.
The man who won his freedom
Francois Bizot, a young French ethnologist, was captured by the Khmer Rouge in 1971. A fluent Khmer speaker, he was accused of being an American spy, and taken to a Cambodian jungle camp where Duch was in charge. During the next three months, as Bizot tried to convince Duch he was not a spy, he got to know him well. Eventually he succeeded in convincing him and became the only foreigner to leave Khmer captivity alive. In The Gate, the extraordinary memoir he wrote about the experience, Bizot wrote: "Duch was the only card I had to play, and somehow I trusted him. Of course he would have killed me if the order came... but only after genuinely trying to save my life. This terrible man was not duplicitous: all he had were principles and convictions." Bizot writes of Duch's "passionate pursuit of moral rectitude". Duch, he said, "was one of those pure, fervent idealists who yearned above all for truth".