From diplomat to detective, this man helped bring Asia's notorious 'Serpent' killer to justice

Updated 0008 GMT (0808 HKT) March 15, 2021

(CNN)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.