The chief interrogator of Cambodia's Khmer Rouge has wept while visiting a prison he commanded where at least 14,000 people were killed.
Duch (centre) was asked to talk through Tuol Sleng's daily routine
Kaing Geuk Eav, also known as Duch, cried during the visit to the S-21 prison, a day after he wept while visiting a mass grave at Choeung Ek.
The visits were led by judges from a UN-backed tribunal which has charged Duch with crimes against humanity.
The Khmer Rouge are blamed for more than one million deaths in the 1970s.
Duch is the first of five senior Khmer Rouge officials to be charged by the tribunal, but a date for the trial has yet to be set.
Both visits, described by officials as re-enactments, were closed to the public and the media, but a witness told the BBC that Duch cried on Wednesday when touring S-21, also known as Tuol Sleng.
A number of survivors - of only a handful to have left the prison alive - also wept as they took part in the visit.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Brutal regime that did not tolerate dissent
More than a million people thought to have died from starvation, overwork or execution
Tuol Sleng is now a genocide museum and is normally one of the busiest tourist attractions in Phnom Penh, says the BBC's Guy De Launey in the Cambodian capital.
For Duch's visit with dozens of investigating judges, police cordoned off the museum and the surrounding area.
Tuol Sleng was once a school, but the Khmer Rouge surrounded the outside with barbed wire and turned the classrooms into tiny cells and blood-spattered torture chambers.
Thousands of people were tortured there until they admitted to crimes against the revolution.
Youk Chhang, the director of the Documentation Center of Cambodia, which researches Khmer Rouge atrocities, said the site was a "living nightmare" for Cambodians.
On Tuesday, Duch wept while touring Choeung Ek, one of Cambodia's notorious killing fields, where some 16,000 people were killed and buried in shallow mass graves after being tortured at Tuol Sleng.
Tourists view human skulls at the Choeung Ek killing field
Reach Sambath, a tribunal spokesman, said Duch cried as he "explained what happened ... when he was the chief of S-21", the Associated Press news agency reported.
"We noticed that he was feeling pity, tears were rolling down his face two or three times," he said.
Duch was especially moved, he said, when he stood before a tree with a sign describing how executioners disposed of their child victims by bashing their heads against its trunk.
He is also reported to have cried when confronted by a pile of human skulls.
Duch was arrested and detained in July 2007.
Those also facing charges include Nuon Chea, second-in-command of the late Khmer Rouge leader Pol Pot, the former foreign and social affairs ministers Ieng Sary and Ieng Thirith, and former head of state Khieu Samphan.