Police have concluded that carbon monoxide killed three high school students at a Gangneung guesthouse. 

The National Forensic Service found much higher-than-lethal levels of carbon monoxide in their bodies, and no other toxic substances.

Police said autopsies will not be performed in accordance with the wishes of their parents.  

Investigators say the main body of a boiler and the vent pipe inside their guesthouse room were faultily connected, possibly causing a gas leak.

The three students were found dead Tuesday at the two-story house in Gangwon Province. Four others were found unconscious and are being treated at a hospital.