Last month prosecutors recommended to the tribunal that five former Khmer Rouge leaders stand trial on charges of "crimes against humanity, genocide, grave breaches of the Geneva Conventions, homicide, torture and religious persecution".
But formal charges have not been filed against Duch, 65, who has been in a military prison since May 1999.
 
He faced investigating judges at a closed-door meeting attended by his
lawyer and translator, a spokesman for the joint Cambodian-United Nations tribunal said.
 
The Khmer Rouge used the S-21 or Tuol Sleng prison to torture suspected enemies before taking them out to the infamous "killing fields" near the city to be shot.
 
Initial interview
 
Reach Sambath, a tribunal spokesman, said the judges would first conduct an initial interview with Duch before deciding on further action.
 
The international panel, called the Extraordinary Chambers in the Courts of Cambodia, was set up to investigate crimes committed during Pol Pot`s reign.
 
The Khmer Rouge`s policies caused some two million Cambodians to die of hunger, disease, overwork and execution.
 
At least 14,000 people deemed to be opponents of the Khmer Rouge were imprisoned at what is now the Tuol Sleng Genocide Museum.
 
Fewer than a dozen are believed to have lived to tell the tale.
 
Survivors
 
One of them, Chum Mey, was delighted to hear that the former prison chief was being questioned.
 
"I want to confront him to ask who gave him the orders to kill the Cambodian people,`` the 77-year-old said on Tuesday.
 
"I want to hear how he will answer before the court, or if he will just blame everything on the ghosts of Pol Pot and Ta Mok."
 
Ta Mok, Pol Pot`s former military chief, died in 2006. Pol Pot died in 1998.
 
Nuon Chea, the Khmer Rouge`s chief ideologue, Leng Sary, the former foreign minister; and Khieu Samphan, the former head of state, are alive but in declining health.