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The smell inside the morgue was overpowering, as disinfectant masked the odor of decaying corpses.

“It’s them,” said a dentist, who had just inspected the mouth of a stiff body.

Light from a window at the back of the room illuminated who she was talking about: two badly burnt bodies that had been opened for an autopsy and stitched back together with surgical cable. The woman’s brain had been bashed in with something heavy and the man strangled, a pathologist said. Both were still alive when they were set alight.

The scene at the police mortuary in Thailand’s capital, Bangkok, on March 3, 1976, remains clear in the mind of former Dutch diplomat Herman Knippenberg. He says it was the most shocking thing he saw in 30 years of foreign service, and sparked a decades-long personal endeavor to bring the alleged killer to justice.

“I had the feeling that I was stepping outside of myself – that I’m on the side, watching the scene,” he recalled in an interview earlier this year.

Knippenberg would later learn the Dutch couple in the morgue were among at least a dozen people Charles Sobhraj admitted to killing – though he later recanted. “The Serpent,” a new BBC/Netflix drama series coming to the streaming service in April, tells how for years, Sobhraj evaded the law across Asia as he allegedly drugged, robbed and murdered backpackers along the so-called “hippie trail” – and how for years, Knippenberg worked with authorities to capture him.

Sobhraj is now serving a life sentence in a Nepalese jail for killing two tourists in 1975. But many of his alleged murders remain unresolved – and for Knippenberg, the case still doesn’t feel completely closed.

Charles Sobhraj in Paris in 1997, following his release from an Indian prison after 21 years.

A fateful letter

In 1976, Bangkok hadn’t yet developed into the metropolis of towering skyscrapers it is today. The subway and Skytrain were yet to be built and bumper-to-bumper traffic meant it could take hours to travel across the hot, crowded city.

Unlike today’s era of instant communication, it was a slower, less connected world. There were no smartphones or social media, and a missing traveler could go unchecked for weeks, maybe even months.

On February 6 that year, Knippenberg received a letter about two Dutch backpackers who had done exactly that.

It was from a man in the Netherlands who said he was searching for his missing sister-in-law and her boyfriend. Henricus Bintanja and Cornelia Hemker had been “ardent correspondents,” writing to their family twice a week as they traveled Asia, the letter writer said. But for six weeks, the family had heard nothing.

Herman Knippenberg in 1975.

“I thought, ‘That is quite bizarre,’” said Knippenberg, who was 31 at the time and a junior diplomat at the Dutch embassy.

Weeks before, two charred bodies had been found on the roadside near Ayutthaya, about 80 kilometers (about 50 miles) north of Bangkok. They had initially been reported as a pair of missing Australian backpackers – until that couple turned up alive. Now, Knippenberg wondered if they were the Dutch couple mentioned in the letter.

So he mobilized a Dutch dentist based in Bangkok to assess the burnt bodies at the police morgue, using the missing couple’s dental records. The dentist was unequivocal: it was a match.

As Knippenberg thought of the mutilated bodies, he remembered a strange story his friend Paul Siemons, an administrative attache at the Belgian embassy, had told him a few weeks earlier – a French gem dealer named Alain Gautier had apparently amassed a large number of passports in his Bangkok apartment belonging to missing people who had allegedly been murdered. Two of the passports were said to be Dutch, but Siemons refused to reveal the source of his information.

Cornelia Hemker (left) and Henricus Bintanja went missing in Thailand in 1975. Their bodies were found burned that year.

At the time, Knippenberg thought his friend had lost it. The story seemed too outlandish.

But as both men would later discover, Alain Gautier was one of multiple aliases used by Sobhraj.

On the run and posing as a gem dealer in Bangkok, the French thief, conman and killer had for years been befriending travelers – then drugging and robbing them. In a time of laxer border security, he often adopted his victims’ identities and used their stolen passports to zigzag across Asia.

Searching for ‘the Serpent’

The day after his trip to the morgue, Knippenberg called Siemons and demanded to know where he’d heard about the gem dealer. After some persuading, Siemons gave him a name – Nadine Gires, a Frenchwoman who lived in the same Bangkok apartment building as Sobhraj, and who introduced clients to him.

Upon meeting Knippenberg, Gires revealed how other people working for Sobhraj had fled after finding a collection of passports belonging to missing people, fearing he’d killed them. She also said she remembered seeing the Dutch couple come to his home.

Knippenberg alerted the Thai authorities, but also continued his own inquiries.

  • Charles Sobhraj’s life of crime

  • Born in Saigon, Vietnam.

  • Jailed for burglary in Paris, France. Serves three years.

  • Returns to prison in France for car theft and evading police.

  • Allegedly commits first murder: a taxi driver in Pakistan. Jailed in Afghanistan for robbery but escapes by faking illness then drugging guards. Jailed in Iran and serves 12-month sentence.

  • Jailed in New Delhi, India for armed robbery. Escapes prison, faking illness.

  • Jailed in Athens, Greece for robbery but escapes by setting fire to a prison van. Allegedly murders at least six people in Thailand, and two in Nepal.

  • Allegedly murders two people in India. Arrested in New Delhi on robbery and murder charges.

  • Sentenced to 12 years in Delhi’s Tihar Prison.

  • Escapes from Tihar by drugging guards. Recaptured in Goa, sentence extended.

  • Released from Tihar and returns to France.

  • Jailed in Nepal on life sentence for 1975 murder.

  • Convicted in Nepal of second 1975 murder.

Source: ‘The Life and Crimes of Charles Sobhraj’ by Richard Neville and Julie Clarke, Reuters

On the morning of March 11, 1976, Gires had some bad news for Knippenberg: Sobhraj and his girlfriend Marie-Andrée Leclerc, a Québécoise also known as Monique, were planning to go to Europe for some time.

Knippenberg told the police and, that evening, officers stormed Sobhraj’s apartment.

They took him in for questioning but the killer was prepared, according to “The Life and Crimes of Charles Sobhraj,” a biography by journalists Richard Neville and Julie Clarke based on hours of interviews with him. Using a passport stolen from one of his victims, which he’d inserted his own photograph into, Sobhraj claimed to be an American citizen and was released from custody.

The following night, an upset Gires called Knippenberg. One of Sobhraj’s housemates, and suspected accomplice, had invited her to the apartment, saying he needed to talk. Knippenberg was torn – if Gires went, it could put her life in danger. If she didn’t, Sobhraj might suspect she had been involved in the raid. “That was one of the most harrowing moments of my life,” Knippenberg said. He thought for a moment, then called her back. “I’m terribly sorry,” he recalled saying. “You have to go.”

While the associate was out of the room, Gires spotted some passport photos and slipped them into her bra – material that gave them more information about one of the victims.

The next morning, Sobhraj and Leclerc left Thailand for Malaysia. It wouldn’t be the last time he slipped through their fingers – a propensity that would later earn him the nickname of “the Serpent.”

Murder on the hippie trail

Born in 1944 in French-administered Saigon to a Vietnamese mother and Indian father, Sobhraj experienced a difficult childhood, according to his biographers. A few years after his birth, his parents split up and he was rejected by his father.

His mother married a French soldier and the family moved to France, where the teenage Sobhraj struggled to settle before entering a life of crime.

Those who met Sobhraj paint a consistent picture of a handsome, charming conman, who had a string of girlfriends – sometimes at the same time. He admired the nihilist philosopher Friedrich Nietzsche and was widely reported to be a martial arts expert.

First jailed in Paris in 1963 for burglary, he’d gone on to escape from prison in several countries, racking up crimes from the Balkans to Southeast Asia. Along the way he enlisted many accomplices, often travelers, his cultivation of a criminal “family” leading some press reports to later label him “Asia’s Charles Manson.

According to his biographers, Sobhraj eventually admitted to at least 12 killings between 1972 and 1976, and hinted at others to interviewers before retracting the confessions ahead of further court cases.

Some of the alleged victims were drugged until they overdosed, some were drowned, while others were stabbed and set alight with gasoline, their bodies burned beyond recognition and dumped by the roadside.

His true number of victims is unknown and only two of the killings ever resulted in murder convictions that stuck.

The first killing he confessed to, according to his biographers, was a Pakistani taxi driver in 1972. But it is in Thailand where his alleged murder spree ramped up. At least six victims – an American tourist, a Turkish man, two French nationals and the Dutch couple – are alleged to have been murdered by Sobhraj and his accomplices there in 1975.

The discovery that year of the dead American woman in a swimsuit, floating off Pattaya beach, would earn him another nickname: “the Bikini Killer.”

Inside Sobhraj’s lair

But Knippenberg didn’t know all that yet.

Sobhraj’s escape left the diplomat feeling depressed. He was fielding angry calls from officials in the Netherlands, who were frustrated at the inaction of the Thai police. Noticing Knippenberg was still working on the case, the Dutch ambassador ordered him to take three weeks’ leave.

Before he left for his holiday, Knippenberg and his then wife, Angela, compiled documents relating to the case – what he now refers to as the “Knippenberg cache” – and dropped them off at embassies around Bangkok.

When he returned, Knippenberg received a call from the Canadian ambassador. Canadian police had visited Leclerc’s parents, who said their daughter had been traveling with her boyfriend and had left an emergency contact near Marseilles, France. When French police checked, they found it was the contact for Sobhraj’s mother.

Charles Sobhraj following his arrest in Delhi in July 1976.

Now they knew the true id