Now we know why this terrifying incident may have occurred. According to a preliminary investigation by China’s aviation authority, a co-pilot on the flight caused it by trying to cover up the fact that he was smoking an electronic cigarette.

As reported in the Chinese business outlet Caixin, the Civil Aviation Administration said the co-pilot had tried to turn off a circulation fan so that the fumes from his vaping would not go into the cabin. Instead, he turned off the supply of air from outside the plane, leading to an oxygen shortage that set off the emergency reaction.

“At present, we are investigating the cause in greater detail, and if the investigation proves it is true, we will handle it according to the law and regulations and deal with it seriously,” said Qiao Yibin, from the aviation authority’s safety office, as quoted by the South China Morning Post.

The smoking of e-cigarettes, like proper cigarettes, is banned on Air China flights. The national carrier said that, if the allegations about its crew smoking were true, it would deal with them with “zero tolerance.”

According to the South China Morning Post, some aviation experts reckon the airline should not have continued with the flight after its emergency oxygen supply had been used up.