At first, nobody guessed the deflated yoga ball in the boot of the Mini Cooper had anything to do with the deaths of Wong Siew Fing, 47, and her 16-year-old daughter, Lily Khaw Li Ling.

Mother and child were found slumped in their parked car one day in May 2015, and both died from carbon monoxide poisoning.

For months police investigators were left without leads. All tests found nothing wrong with the car. To completely rule out a malfunction of the vehicle's ventilation system, officers, with the help of Interpol, even asked staff at the car's maker, BMW, in Germany, to do further tests.

The Mini Cooper where the deaths occurred. (Photo / Supplied)

As they waited for the results to come back, detectives spotted the vital clue everyone had overlooked: the deflated grey yoga ball in the boot, its stopper nowhere to be found.

"Why is it sitting there without a plug?" a detective wondered aloud.

That was what eventually led to police uncovering the bizarre murder plot of Malaysian university professor Khaw Kim Sun - Wong's husband, and the girl's father.

Khaw, 53, an associate professor of anaesthesiology at Chinese University, was found guilty of murder on Wednesday, after a unanimous jury believed the prosecution's case that he planted the leaking, gas-filled ball in his wife's car.

Now it can be revealed how the detective and his colleagues came to solve one of Hong Kong's most intelligent murder cases in recent times, and the challenges it entailed. The officer preferred to remain anonymous as he was not authorised to speak on the record.

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"This was way out of my repertoire," he said, recounting a highly technical investigation that involved interviews with more than 50 witnesses ranging from doctors to chemical suppliers to psychologists.

The officer admitted an initial concern: he and his colleagues were up against someone whose friends and co-workers later described him in court as a "clever" and "avant-garde" anaesthesiologist. The officer recalled a sense of cluelessness on the day the pair died.

Police went to Khaw's home that day to seize everything they could in the kitchen - even flour.

"Who knows? He's an anaesthesiologist," the officer said.Prompted by what they had found in the boot, a few feet from the victims' bodies, they had an idea: that the ball could have been filled with the noxious gas and left to deflate in the car. To test if the theory worked, an officer bought two similar yoga balls to do some experiments. They had assumed the yoga ball, like a beach ball, would have some mechanism to stop it deflating even if the plug was out. But they discovered it did leak without the stopper.

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Experts later confirmed yoga balls were capable of carrying carbon monoxide, in theory at least, prompting police to seize the ball officially in November, six months after the alleged murder.

"It only happens in TV dramas," the officer recalled thinking.

Their investigation led them to one of Khaw's colleagues, who had a quotation form Khaw sent to a carbon monoxide supplier, asking about prices.

Sixteen-year-old daughter, Lily Khaw Li Ling. (Photo / Supplied)

The colleague, Dick Chow Ho-kiu, later bought a tank of the gas at Khaw's request and helped him conduct experiments on rabbits. Chow told police that after one experiment he saw Khaw take the carbon monoxide home in two yoga balls.

"This just proved we were right," the officer said.

A possible motive had also revealed itself as police dug deeper: Khaw was having an affair with a student he used to mentor, Shara Lee, now an assistant professor at Polytechnic University. His relationship with his wife was more like a business partnership; they stuck together only to raise their children, the trial heard.

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The investigation team arrested and charged Khaw on September 11, 2017.

Khaw, who taught at the department of anaesthesia and intensive care, was the kind of person "who knows a lot" and "knows how to take care of things", according to friends who testified during the trial.

His family of six, including three daughters and one son, lived in the tranquil neighbourhood of Tai Tung Village in Ma On Shan when the 2015 tragedy struck.

On May 22, Wong left home with Lily at about 2pm in the yellow Mini Cooper to pick up her other children from school.

After travelling 1.6km down Sai Sha Road - about four minutes - Wong parked the car at the bus stop, and was first spotted by a bus driver 25 minutes later.

At 2.14pm, Lily - who the court heard was a girl "full of life" - had sent a text message to her friend Sarah Niu. It simply read: "Congratulations." It was a message of joking jealousy, after Niu got out of doing an assessment at school.

Both Wong and her daughter were found unresponsive in their car two hours later by a jogger and others, who called an ambulance for them.

It was too late for doctors to save their lives when they arrived at Prince of Wales Hospital in Sha Tin, the very hospital Khaw Kim Sun worked at.

But when Khaw's trial came to an end, some mysteries remained - the biggest being how the yoga ball ended up in the car.

There were security cameras installed in the family home, yet they were not working around the time of the killing.

While the professor denied committing murder, he did not dispute taking the gas home.

He claimed he took it back to kill rats infesting his village house. He suggested Lily somehow might have used it to commit suicide.

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His lawyer, Gerard McCoy SC, presented an alternative version in court, saying Lily, who had a phobia of bugs, used the gas to kill insects, unaware of the potential fatal consequences.

The defence also argued Khaw had an alibi. With a concentration so high it could cause someone to pass out in minutes, the defence argued. It would require the ball to have been planted right before the car was driven away, still leaking the gas. That would put Khaw out of the picture, as he left home at about noon for a university seminar.

Prosecutor Andrew Bruce SC, however, suggested there was sufficient gas in the ball and the car on the day that, even if the ball was placed in the vehicle an hour or two before, it would still maintain a high enough concentration to kill.

Khaw Kim Sun. (Photo / Supplied)

A report presented to the court suggested the concentration of carbon monoxide inside the car reached at least 7000 parts per million (ppm) at one point, lasting more than two hours. It takes 1600ppm of carbon monoxide to make someone feel dizzy in just 20 minutes and die in an hour, according to medical literature.

Dr Lau Fei-lung, chairman of the clinical toxicology board at the Hong Kong College of Emergency Medicine, said it would be hard to pinpoint at what moment the ball was placed in the car, as it involved too many variables. Both suggestions from Bruce and McCoy could make sense, he said.

The closest the trial could get was that, when their helper Siti Maesaroh saw Wong and Lily head towards the car that day, they were not carrying a yoga ball.

She failed to recall it when giving evidence in court, but a statement the helper gave to the police said she saw Khaw leaving the house for the car park using a less commonly used door at about 11am.

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But Lau was sure of one thing.

"The concentration in the car must have been very high," he said, as it took only a few minutes for them to feel unwell.

The high levels would have made the mother and daughter feel woozy very quickly, which explained why Wong pulled over and spared a traffic accident. This also resulted in a fatal delay to their discovery, as they remained in the car, inhaling more gas, the court heard.

For our anonymous police officer, the lack of a crash at least provided him and his colleagues with an intact crime scene and first access to it.

The concentration in the car must have been very high Lau Fei-lung, toxicology specialist

"If it had resulted in an accident, it might have been handled by the traffic police," the officer said.

Another unsolved mystery was why Khaw killed his daughter, who the court heard he had bonded well with, despite burdening her with high expectations.

Prosecutors suggested perhaps Khaw never intended for her to die. In fact, he did warn Lily not to go out that day.

Oliver Chan Heng-choon, an associate professor of criminology at City University, said that although the murder plan was intelligent, it was far from sophisticated, mainly due to the careless trails Khaw left behind.

For instance, being spotted smuggling the gas home by Chow, his colleague. A plastic plug which could fit the yoga ball in question was also recovered from a drawer in his bedroom.

But forensic scientists had no way to prove whether this small piece of plastic, which guided police in the right direction, did actually belong to the ball. Just as no one could tell when the yoga ball was placed in the car.

Only Khaw knows.