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

(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.

"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.

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 identity of Leclerc's boyfriend: he was Charles Sobhraj.

That month, Gires called, warning Sobhraj's landlord planned to rent out his Bangkok apartment and throw away his belongings. Concerned crucial evidence would be lost, Knippenberg rallied a team and descended on the condo.

It was "seedy and filthy," Knippenberg remembers. They found 5 kilograms (11 pounds) of medicine and three industrial-size cartons of liquid containing a drug that acted as both a laxative and a "chemical straitjacket," Knippenberg said. They also found the Dutchwoman Hemker's coat and handbag.

On May 5, 1976, the Dutch ambassador told Knippenberg to share the story with the press. Within days, the Bangkok Post printed an explosive front-page story headlined: "Web of Death."

After that, the Thai authorities took notice. They issued an Interpol notice -- and that, says Knippenberg, helped lead to Sobhraj being captured in India on July 5, 1976.

Web of Death: Reports of the murders hit the Bangkok Post's front page on May 8, 1976.

Sobhraj's life behind bars

Not for the first time, Sobhraj was on the run.

By the spring of 1976 he was back in France. But with the so-called "bikini murders" now making international headlines, he fled to India with Leclerc -- arriving in New Delhi by early June that year after driving overland in a Citroën CX 2200, according to his biography.

The international arrest warrant put Sobhraj on the authorities' radar -- and the Indian police had their own bones to pick with him.

Indian authorities arrested Sobhraj after he bungled the drugging of a French tour group in New Delhi in July 1976. He was also charged with the killings that year of an Israeli man in Varanasi and a French tourist in Delhi.

While his convictions for those two deaths were later overturned on appeal, he was found guilty of trying to rob the tour group and sentenced to 12 years in the Indian capital's notoriously overcrowded and understaffed Tihar Prison.

Life behind bars wasn't all bad for Sobhraj. Sunil Gupta, a former superintendent and legal officer at Tihar, says he enjoyed special privileges -- including food made according to his preference and conjugal visits not usually afforded to inmates.

"Prisoners were supposed to stay in their wards but he would roam around freely," says Gupta, author of "Black Warrant: Confessions of a Tihar jailer," a memoir of his more than 30 years working at the Delhi prison.

Sobhraj is led to Delhi's Tihar Prison in April 1977. He enjoyed special privileges behind bars, according to one of his jailers.

According to Gupta, Sobhraj earned money by drafting court petitions for wealthy inmates, and then maintained his elevated status by bribing guards. He was also said to have made secret recordings of senior prison officials that would implicate them in corruption. "Everyone was scared of him," Gupta says.

When Bangkok-based journalist Alan Dawson interviewed Sobhraj at Tihar in 1984, he noticed he "seemed to have the run" of his section -- in what he said was a "horrible prison, with thousands of family members, lawyers, shysters and others clamoring for a word with their prisoner."

"Tihar was an eye-opener to me," Dawson said via email. "The prisoners ran life inside the walls and bars, and the 'authorities' handled the paperwork and so on.

"Even by those standards ... Charles was a bit of a revelation. He had a suite of three cells, and the prison warden -- he introduced us -- called him Mister Charles. I was whisked through the front gate security, and it seemed the guards had instructions to be nice to me. Whether the instructions came from the warden or Charles ... who knows?

"From the very start, it was obvious to me that Charles was a conman, seeking control of the situation. He was a good-looking guy, and had that swindler's knack of making you believe you were the center of his attention."

Another prison break

On March 17, 1986, Sobhraj pulled off one of his biggest swindles yet.

Gupta says he was watching a movie at home when a breaking news announcement cut in: Sobhraj had escaped from jail. Gupta hurried to the prison where he found a shocking scene: all the gatekeepers were asleep. Sobhraj had told staff it was his birthday and given them sweets laced with sedatives. More than a dozen prisoners escaped.

Sobhraj had just weeks to go until his release -- but Gupta suspects he was worried about being extradited to Thailand, where he faced murder charges for the 1975 killings punishable by death.

Connie Jo Bronzich (left) and Laurent Carrière were killed by Sobhraj in Nepal in 1975. Decades later, he was convicted of both murders.

Thousands of miles away in the United States, Knippenberg was studying for a master's degree in public administration at Harvard University when he received a call from his program adviser.

"I think you should go underground for the time being," she told him. "Sobhraj has escaped from Tihar jail and I think your life may be in danger."

Knippenberg was skeptical -- he believed Sobhraj would be too smart to come after him and would be hiding in plain sight.

He was right. Sobhraj was caught on April 6 "while he sipped beer in the seaside resort of Goa to celebrate his 42nd birthday," as the Associated Press reported at the time. "He didn't say anything. He went quite coolly," said Gines Viegas, the owner of the Coconut Tree restaurant where Sobhraj was captured, according to the report.

He was jailed for an extended sentence, during which the statute of limitations on the alleged Thai murders would expire. Sobhraj no longer faced almost certain execution.

One big question

Sobhraj has never given a convincing reason for the murders.

Dawson, the journalist, had planned to write a book with the killer, but said he abandoned the idea when Sobhraj demanded $10,000 to cooperate. Nevertheless, he continued with the interview in their 1984 meeting at Tihar jail. The first question: "Why?"

"Well, he never had a good answer," Dawson says. "He implied that if 'we' wrote a book, then the answer would be that all those white people had corrupted and ruined Asia by trafficking opium.

"And therefore, his reasoning was that today's white people deserved to die for it."

Sobhraj prays at Notre-Dame Cathedral, Paris in 1997 after being released from jail in India.

Describing his meetings with Sobhraj, author Neville wrote he initially had "a crude theory of Charles as a child of colonialism revenging himself on the counter culture. Instead, I was dazzled by a brilliant psychopath."

According to Neville, Sobhraj explained the murders by saying "I never killed good people," and drew from "psychoanalysis, global politics, and Buddhism, to create a cozy world of rationalization and extenuating circumstances," to justify his crimes.

"His claims that his life was a protest against the French legal system or that his love for Vietnam and Asia motivated his criminal career are absurd, but as tools of psychological manipulation they were very effective," Neville wrote.

Asked by Neville what makes a murderer, Sobhraj replied: "Either they have too much feeling and cannot control themselves, or they have no feelings. It is one of the two."

The killer did not say which of the two applied to him.

Sobhraj had "always wanted his name to be in the spotlight," according to Gupta, his jailer. But upon his release from Tihar in 1997, after 21 years locked up, his media presence amplified.

The killer sold the movie and book rights to his story for $15 million to an unnamed French actor-producer, according to the BBC, though the film was never made.

Despite several books and numerous television shows about Sobhraj, Dawson says we still don't know the true motives for his "terrible, murderous violence."

"It's why I went to Delhi to see him and here I am (more than) 35 years later and still (have) no real clue," he said.

Murder convictions

On a 2003 winter's morning in Wellington, New Zealand, Knippenberg was marking his first day of retirement with pancakes. Once again, there was a fateful phone call from a friend -- Sobhraj, who had been living in France, had just been arrested in Nepal and charged with the 1975 murder of a tourist in Kathmandu.

Sobhraj's decision to travel to Kathmandu was a curious choice: Nepal was the only place in the world where he was still a wanted man. Under questioning from Nepalese police, Sobhraj denied he had ever previously visited the Himalayan country.

Sobhraj leaves Kathmandu district court after a hearing on May 31, 2011.

Knippenberg went down to his garage where there were six boxes of documents related to the Sobhraj case. As he fished out the statement Leclerc had made when she was captured in July 1976, Knippenberg found he had remembered correctly: Sobhraj's former girlfriend had described in detail the time she spent in Nepal with him.

He sent those documents to the FBI.

"I think it goes too far to say that I was directly responsible for his conviction in Nepal," Knippenberg says. "Though my efforts indicated to Nepal police what there was and where to look for it."

Sobhraj was arrested in the Nepalese capital on September 13, 2003, and charged over the 1975 murder of American tourist Connie Jo Bronzich. He professed his innocence.

Sobhraj, face covered, is swarmed by photographers outside Kathmandu district court on September 22, 2003.

But, as Sobhraj's lawyers detailed in a complaint filed with the UN Human Rights Committee in 2008, his arrest and trial allegedly breached his human rights. Sobhraj was detained for 25 days without a lawyer, then sentenced in August 2004 to life imprisonment -- even though he hadn't been able to call his own witnesses or hear evidence presented against him as he couldn't speak Nepalese. The document said he had been kept almost continuously in isolation.

In a 2010 opinion piece, the then officer-in-charge of the office of the UN High Commissioner for Human Rights in Nepal, Anthony Cardon, wrote human rights should be afforded to everyone, "however notorious their ... alleged crimes."

It made no difference. Sobhraj remained in jail, losing several appeals.

In 2014, a Nepalese court convicted Sobhraj for the 1975 murder of Canadian tourist Laurent Carrière, handing down a 20-year sentence. The case was reopened in 2013 because prosecutors were concerned Sobhraj might appeal for an early release from prison due to old age, according to a Nepalese court official.

Behind bars, Sobhraj still made headlines. In 2008, then age 64, he married his lawyer's 20-year-old daughter, Nihita Biswas, who also acted as his translator. "He's innocent," Biswas said in a Times of India interview that year. "There's no evidence against him."

Sobhraj's wife, Nihita Biswas, speaks with media outside the Supreme Court in Kathmandu on July 30, 2010.

Never truly over

In some ways, the case is now settled. Sobhraj, 76, is serving a life sentence. Many of his alleged accomplices are missing, or dead.

When he reflects on the case that absorbed the better half of his life, Knippenberg, also 76, believes it got under his skin because he saw injustice. "I was confronted with a situation in which innocent people were losing their lives and nobody lifted a finger," he said. "I saw that as the complete failure of democracy."

Herman Knippenberg with actor Billy Howle, who plays him in BBC/Netflix series "The Serpent".

That obsession has impacted his life at times -- his fixation on the case has sometimes made his workmates view him as a bit of an oddball, he said. But in the BBC/Netflix drama released this year, which Knippenberg consulted on, the former diplomat is painted as a hero. He acknowledges the information he provided helped get Sobhraj arrested in two countries, but says he doesn't think of himself that way.

"I do not see any heroes here. It was a tragic misuse of the supremely gifted mind," he said, of Sobhraj.

More than 45 years after that fateful letter, Knippenberg said he wouldn't be surprised if he read tomorrow that the Nepalese government had decided to let Sobhraj go.

True resolution, he said, can come only one of two ways.

"This isn't over for me until he is in a better world, or I am in a better world," Knippenberg said. "I don't take anything for granted."

CNN's Esha Mitra contributed reporting from New Delhi.