BANGKOK, Thailand — The former chief torturer for the Khmer Rouge became the first man to be charged yesterday with crimes against humanity over the Cambodian genocide, three decades after his government killed 1.7 million of its own people.

Kang Kek Iue, 65, who is usually known as Duch, was transferred from a military prison to the custody of the United Nations-backed genocide tribunal in Phnom Penh and questioned by judges.

For three years up to 1979, the former mathematics teacher ran the S–21 torture center, meticulously documenting his victims with black-and-white photographs that now stand among the defining symbols of 20th-century barbarity.

Duch does not deny his work at the prison, but his lawyer protested that he had merely been following “verbal orders from the top.”

The Maoist Khmer Rouge seized power in 1975 and declared “year zero” as they set about dismantling urban society to build an agrarian utopia.

From 1976, more than 16,000 prisoners were sent to Duch. By the spring of 1977, about 1,000 people were being “smashed” every month. In 1978, as the paranoid government purged sections of the army, S–21 was barely able to cope with the influx. “There is no need to interrogate them, just smash them,” Duch was allegedly told by Nuom Chea, the regime’s so-called Brother no. 2. “No such order had ever been received before,” the punctilious torturer noted with disapproval. “Nor were we used to working in that way.”

Historians believe that the raison d’être of S–21 was torture, extracting thousands of pages of detailed confessions as “proof” to justify the regime’s purges.

John Dewhirst, a schoolteacher who was one of the regime’s two British victims, confessed that he had joined the CIA at the age of 12. Duch later recalled that he was “very polite.”

If they survived long enough, prisoners were taken outside the city and beaten to death with iron bars.

“Duch set aside specific days for killing various types of prisoners: one day, the wives of ‘enemies’; another day, the children; a different day, factory workers,” Elizabeth Backer wrote in her book “When the War Was Over.”

Only about a dozen inmates are believed to have survived their ordeal, of whom perhaps only four are still alive now.

Among them is Chum Mey, 77, who expressed his delight at yesterday’s development. “I want to confront him to ask who gave him the orders to kill the Cambodian people,” he said.

The crimes committed at S–21 were only a small part of the Cambodian genocide. Prosecutors have prepared cases against four other Khmer Rouge leaders, who have not been named but may include Nuom Chea, Ieng Sary, the former foreign minister, and Khieu Samphan, who was head of state.

All of them are living freely in Cambodia but in declining health, leading many to fear that they could die before they face justice. Pol Pot, who was Brother no. 1, died in 1998 and Ta Mok, sometimes known as “the Butcher,” in 2006.

The tribunal itself has faced numerous delays caused by disputes between the foreign and Cambodian lawyers and judges involved. It is already almost half way through its designated three-year period of existence.

Duch himself could easily have missed his day in court. In 1979, as the Vietnamese troops who overthrew the Khmer Rouge advanced on Phnom Penh, he killed the last prisoners and fled.

Until he was tracked down by Nic Dunlop, an Irish photojournalist, in 1999 many assumed he was long dead. In fact he had returned to his old profession as a teacher and converted to Christianity.

Christopher LaPel, who baptized him, later recalled: “Before he received Christ, he said he did a lot of bad things in his life.

“He said, ‘Pastor Christopher, I don’t know if my brothers and sisters can forgive the sins I’ve committed against the people.'”