An anesthesiologist used a yoga ball filled with carbon monoxide to kill his wife and daughter in 2015, prosecutors told a Hong Kong court Wednesday.

Khaw Kim-sun allegedly left the exercise ball filled with the gas in a locked car with the two women, where it slowly leaked carbon monoxide and killed his wife and 16-year-old daughter, the BBC reported. Khaw pleaded not guilty to “two counts of murder.”

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An autopsy determined the doctor’s wife and daughter died from carbon monoxide poisoning. While investigating, police discovered a deflated yoga ball in the car near where the two bodies were found. The South China Morning Post reported a jogger found the bodies in the parked car.

Khaw, who was an associate professor at the Chinese University of Hong Kong, allegedly killed his wife because he was having an affair with a student and his wife wouldn’t divorce him, prosecutors told Hong Kong’s High Court, according to Sky News. However, prosecutors said they didn’t think Khaw wanted to kill his daughter.

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Witnesses reportedly said they saw Khaw filling exercise balls with carbon monoxide. The professor told colleagues he was using the gas to kill rabbits, however, he allegedly contradicted his statement when speaking to the police, saying he was using the carbon monoxide to kill rats in his residence.

The trial was slated to continue on Thursday.