A 53-year-old professor at the Chinese University of Hong Kong denied murdering his wife and daughter after leaving a yoga ball filled with carbon monoxide inside his wife’s car.

The defendant, Khaw Kim-sun, a father of four who taught at the university’s department of anesthesia and intensive care, denied two counts of murder at the High Court on Wednesday, Apple Daily reported.

In May 2015, his 47-year-old wife surnamed Wong and their second child, a 16-year-old girl, were found unconscious in a Mini Cooper that was parked by a bus stop near Sai O Village in the New Territories.

The mother and daughter were taken to hospital, where they were certified dead. Post-mortem examinations found they had both died from carbon monoxide poisoning.

Police found a deflated yoga ball, whose opening was not plugged, in the boot of the car.

On Wednesday’s trial, the prosecutor noted that Khaw had admitted to the police while under caution that he had filled the ball with carbon monoxide at a university’s laboratory two days before the incident. He claimed that he did this in order to kill laboratory mice.

The prosecutor claimed that, as an expert in anesthesia, Khaw would know the amount of gas in the ball was enough to kill. Anyone who put a fitness ball full of carbon monoxide inside the confined space of a vehicle would be intent on murder, the prosecutor added.

Khaw knew that his wife would drive the car that day to pick up some of their four children. However, the teenage daughter who died in the incident only rode with her mother as she did not have school that day.

The prosecution argued that, although he did not intend to kill his daughter, Khaw should also be held responsible for her murder.

The court heard that the defendant had discussed divorce with his wife in 2013 after his wife found out that he was having an affair with his research assistant, Sing Tao Daily reported.

Between 2013 and 2015, Khaw and his girlfriend traveled outside Hong Kong at least nine times.

The trial continues on Thursday.