Government officials are baffled over what killed four people last week at a mine in southeastern British Columbia, Canada that has been closed 4-1/2 years, according to The Globe and Mail.

It is the worst accident ever in the century-long history of the Sullivan lead and zinc mine near the resort town of Kimberley. Sullivan was decommissioned in December 2001.

The tragedy began when Doug Erickson, 48, was found dead by Bob Newcombe, 49, on Wednesday morning, May 17. Erickson was an environmental consultant and had conducted monthly water testing in a sampling shed outside the Sullivan mine for several years. When he was reported missing, Newcombe, an employee of Teck Cominco Ltd., was sent to find out what happened.

"They obviously didn't consider it to be a particularly dangerous situation," said Bill Bennett, B.C. minister of state for mining said.

A Teck Cominco spokesman said the shed had not been considered hazardous. The incident remains under investigation.

On finding Erickson, Newcombe called 911, which dispatched two paramedics on what was believed to be an emergency medical call.

Firefighters followed soon thereafter. Believing they would be facing an enclosed-space situation, the firefighters donned masks. They found four bodies. The paramedics were not wearing masks.

In a press release Thursday, Teck Cominco said B.C.'s chief inspector of mines has confirmed the water sampling shed was "oxygen deprived" but the cause of death remains unknown.

People have speculated that the cause is hydrogen sulphide, a deadly gas that is colorless, smells like rotten eggs at low concentrations and kills swiftly. If the gas is heavy, there is no smell and death is almost instantaneous.