Among the black and white portraits of doomed Cambodian men, women and children lining the walls of the Tuol Sleng Genocide Museum , one face stood out. Gaunt and haggard, the bearded young westerner stared into the camera with sunken, despairing eyes — eyes that spoke volumes about the horrors he endured here when this former primary school in a quiet Phnom Penh suburb was known as S-21, the secret Khmer Rouge detention centre where over 17,000 people were detained and tortured before they were exterminated in the nearby killing fields of Choeung Ek.

In just four years (1975-79), the Khmer Rouge, led by a megalomaniacal former grade school teacher named Saloth Sar (Pol Pot was his nom de guerre), murdered over 3 million of their fellow Cambodians. Most died from starvation or disease while toiling in the regime’s agrarian concentration camps; hundreds of thousands more were systematically killed in the regime’s apocalyptic attempt to eradicate all vestiges of modern civilization.

Some prisoners held at S-21 were killed and buried on the prison grounds. Most were herded into trucks and driven to a former orchard called Choeung Ek on the city’s outskirts where they were slaughtered en masse and then tossed into mass graves. To the outside world, Choeung Ek and other extermination centres scattered throughout Cambodia would come to be known as the Killing Fields.

The Westerner in the prison portrait was likely one of two young Americans — Michael Deeds and Chris DeLance — former surfers from Hawaii who made the fatal error of sailing too close to the Cambodian coast one December day in 1978. Seized by a Khmer Rouge patrol boat, they spent time as guests of the Khmer Rouge in S-21, the most notorious detention centre in the country, before their hosts murdered and buried them on the prison grounds several feet from where I now stood.

Were Deeds and DeLance smuggling drugs from Thailand to the Philippines when they were apprehended? Or were they simply unfortunate young adventurers caught in the wrong place at the wrong time who experienced the ultimate backpacker’s nightmare? We’ll never know, but what we do know from prison records is that they were among a handful of westerners from Australia, England, France and the USA were captured at various times by Khmer Rouge patrol boats while sailing off the Cambodian coast.

25 years after the invading Vietnamese forces liberated Tuol Sleng, it remains a ghostly and macabre place. I arrived early by motorcycle taxi, just as the museum opened, to find myself its only visitor. While my teenage driver waited, I wandered across the grassy lawn and entered what were once classrooms where teachers like Saloth Sar once taught French grammar, mathematics, and history to generations of grade-school children. Inside the classrooms, rusted beds, grisly instruments of torture and hundreds of harrowing portraits of those misfortunate enough to end up here evoked images too horrifying to comprehend.

We know from S-21’s captured records that its Western guests were treated no differently than Cambodians. Locked in long open classrooms, they slept among rows of other prisoners shackled to the floor, or were chained inside tiny brick cells. Sadistic guards — often boys as young as 13 — mercilessly harangued and routinely beat them. Prison regulations scribbled in French on dusty blackboards that once contained children’s lessons remain untouched on the classroom walls.

In the smaller classrooms that line one wing of the complex interrogators chained individual prisoners to bed frames for hours, electrocuting them, burning them with cigarettes, pulling out their fingernails, and forcing them to drink urine and eat feces. The goal was to extract confessions confirming what the paranoid Khmer Rouge regime had already concluded — that they were imperialist spies and subversives bent on plotting the destruction of Angkar, the shadowy Orwellian force that ruled Cambodia.

Prison records indicate that foreign captives like Deeds and DeLance likely confessed to being CIA spies engaged in fantastical espionage plots concocted by their interrogators. Yet confessing their imaginary crimes failed to save them: they endured more torture before being strangled and buried on the prison grounds. Deeds was among the last to die at S-21: his confession was dated January 5, 1979, only two days before invading Vietnamese forces overran Phnom Penh and entered the prison to find only seven prisoners still alive. The last shipment of condemned prisoners had left for Choeung Ek only hours before.

I left Tuol Sleng feeling strangely numb, perhaps the only way I could subconsciously process what I had just seen. My driver was waiting, ready to whisk me out beyond the city limits, past fields of rice paddies, and down a series of country lanes that ended at a dusty parking lot that serves as the entrance to the Choeung Ek Killing Fields Memorial.

In 1980, the remains of nearly 9000 people, many of whom were bound and blindfolded, were exhumed from mass graves in this former orchard; 43 of the 129 communal graves here have been left untouched. Human bone fragments are scattered around the disinterred pits. Over 8000 skulls, arranged by sex, are visible behind the clear glass panels of the Memorial Stupa, which was erected in 1988.

The staggering horror commemorated at Choeung Ek is all the more surreal by the bucolic surroundings - tranquil rice paddies, rustic villages, and children playing in the nearby ditches and paddies. As I stood by the side of an open grave I again tried to comprehend the enormity of what occurred here and at S-21 only a quarter of a century ago.

My thoughts kept returning to S-21’s classrooms, with those hundreds of prisoner portraits covering wall after wall from floor to ceiling (each prisoner who passed through S-21 was photographed). I couldn’t begin to imagine what their experiences must have been like there before they met their ends here. The expressions on face after face — fear, confusion, despair — cried out across a quarter century for help, for comfort.

Perhaps because he was, like me, a foreigner in this strange land of beauty and unfathomable suffering, my thoughts also kept returning to that portrait of Michael Deeds; one moment a carefree young adventurer living the packpacker dream so many of us yearn to experience, the next forced to witness firsthand one of humanities darkest chapters before he too became one of the ghosts on the walls of S-21.