Judges at the UN-backed Khmer Rouge tribunal in Cambodia have begun to question the first of the suspects they say could stand trial.
Duch has been in prison since 1999
Kang Kek Ieu, also known as Duch, was in charge of a notorious prison in the country's capital, Phnom Penh.
He ran a facility known as S21, where thousands were tortured and executed.
He is one of five suspects prosecutors have asked judges to investigate over deaths under the Khmer Rouge government in the late 1970s, a spokesman said.
As many as two million people are thought to have died during the four years of Khmer Rouge rule.
Duch was not among the top level of Khmer Rouge leaders but he has become one of its most notorious members, says the BBC's Guy Delauney in Phnom Penh.
There are only seven known survivors of the S21 prison. A museum at the site illustrates in graphic detail what happened to the rest of the inmates.
Many of them were executed at the so-called Killing Fields outside the city.
Judges will question Duch, who has been held in a military prison since 1999, about his role in the events at S21.
"They (the judges) need to do an initial interview with him, but he has not been formally charged yet," said tribunal spokesman Reach Sambath.
The UN-backed tribunal has taken years to get off the ground.
But by questioning Duch, the judges are sending out a clear message that the special courts are now operational and moving more quickly than many people expected, our correspondent says.
Bringing in the other four suspects could, however, be more difficult.
None of them have been named officially but all of the surviving former leaders of the Khmer Rouge have been living freely in Cambodia, our correspondent adds.