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A bright yellow Mini Cooper sat parked near a bus stop in Ma On Shan, a town in Hong Kong.

It was not raining that day in 2015, but the windshield wipers were still running, scraping against the dry glass; and, a passerby later said, it appeared that Wong Siew-fung and her 16-year-old daughter had fallen asleep inside the car.

But 45 minutes later, the same witness said, they had not moved. The mother and daughter were rushed to a hospital, where they were pronounced dead.

The account comes from court testimony reported by the South China Morning Post. Prosecutors told Hong Kong’s High Court earlier this week that autopsies later revealed that the two died of carbon monoxide poisoning, though the gas did not appear to be leaking from the vehicle, according to the South China Morning Post. The newspaper reported that it apparently came from a yoga ball in the trunk — which, prosecutors said, had been filled with the gas and placed there by Wong’s husband.

When police responded to the scene, the ball was lying deflated in the back of the vehicle.

According to the South China Morning Post, prosecutor Andrew Bruce told the court that Khaw Kim-sun, a 53-year-old anesthesiologist, pumped carbon monoxide into the ball, unplugged it and put it into the vehicle in May 2015 as part of a plan to kill his wife but that “the last thing the accused wanted was for his 16-year-old to die.” Khaw apparently did not expect that his daughter would be in the car at the time. Still, the prosecutor said, “if that person knew what was in the car was carbon monoxide and knew it was a dangerous gas likely to kill you, you can confirm this person had homicide on his mind.”

Khaw, who is charged with two counts of murder, pleaded not guilty, according to BBC News.

The South China Morning Post reported that during the second day of trial Thursday, a forensic pathologist told the court that Khaw’s wife and daughter died between 12:45 p.m. and 2:45 p.m. May 22, 2015. Autopsy results showed that Wong, his wife, had a carbon monoxide level of 50 percent in her bloodstream, the expert said; the couple’s daughter, Khaw Li-ling, had a carbon monoxide level of 41 percent in hers.

Prosecutors said that Khaw, a professor at the Chinese University of Hong Kong, had been cheating on his wife with a student and that his wife would not grant him a divorce, according to the South China Morning Post. The newspaper reported that a housekeeper told the court Thursday that the couple had been sleeping in separate bedrooms.

So, the prosecutors said, Khaw, with help from his student, devised a plan to kill his wife.

Bruce told the court that Khaw ordered tens of thousands of dollars' worth of carbon monoxide, claiming that he planned to use it in a science experiment, according to the South China Morning Post. The newspaper reported that when his colleagues saw him pumping the toxic gas into a couple of yoga balls, he apparently told them that he was going to test it on rabbits.

Carbon monoxide is a colorless and odorless but deadly gas that can be found in fumes from fuel, gas stoves, gas fireplaces, according to the Centers for Disease Control and Prevention.

At certain levels, the CDC stated, the gas can be poisonous to people, causing headaches, dizziness, confusion and, ultimately, death.

During the police investigation into his wife and daughter’s deaths, Khaw apparently told investigators that he took the carbon monoxide-filled yoga balls home to attempt to kill rats, but his housekeeper told authorities that were none, according to the South China Morning Post.

The newspaper reported that at one point, Khaw even suggested that his daughter may have tried to use the gas to commit suicide.

The prosecutor called it “a lame lie.”

“It is simply not true,” Bruce told the court, according to the newspaper.

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