Alleged weapon in double murder: A gas-filled yoga ball

He told co-workers he was going to kill rabbits, and he told police that he only meant to get rid of rats—but prosecutors say a Hong Kong professor used a rather odd weapon to instead murder his wife and teen daughter.

That alleged weapon: a yoga ball filled with carbon monoxide, which Khaw Kim-sun is accused of leaving in the trunk of a Mini Cooper his wife, Wong Siew-fung, was driving in May 2015, the South China Morning Post and BBC report.

The ball is said to have deflated while in the car, leaking the gas that killed Wong and 16-year-old Khaw Li-ling; the two were found unconscious in the locked car by a jogger.

Prosecutor Andrew Bruce says Khaw, a Malaysian professor specializing in anesthesiology at Chinese University, was having an affair with a student and that Wong knew about it but refused to grant him a divorce.

Per Coconuts, Bruce told a Hong King jury Thursday that Khaw's anesthetist background gave him knowledge of, and access to, the carbon monoxide, and that the student he was having the affair with helped him with the "research project" he'd set up as a ruse to get his hands on the carbon monoxide.

Bruce adds that co-workers saw Khaw filling two yoga balls with the gas, explaining that he wanted to see how it affected rabbits. He later changed that story and told cops he wanted the carbon monoxide to kill rats at his house, though a worker at his home said rats weren't an issue there.

The one part of his alleged scheme that went awry, per Bruce: Although Khaw meant to kill his wife, he hadn't expected his daughter to be in the car with her.

"The last thing the accused wanted was for his 16-year-old to die," Bruce says, per the Post. Khaw has pleaded not guilty to two murder counts.

More From Newser