VANCOUVER, British Columbia (Reuters) - An accident that killed four people at a decommissioned mine in Western Canada in May was "unprecedented" in the history of mining, according to a government investigation released on Monday.

The victims, including two paramedics, died after they entered a building that had filled with oxygen-depleted air that had apparently come from the tailings dump at the former Sullivan lead and zinc mine in Kimberley, British Columbia.
Investigators had initially thought that poison gas killed the victims, but tests found there was simply not enough oxygen to survive and the victims quickly succumbed.
"We have clearly established the cause of death of the four victims, but this accident is unprecedented in the history of mining and the process that led to the oxygen-depleted atmosphere has not, to our knowledge, occurred anywhere else in the world," said Fred Hermann, the province's chief mine inspector.
Inspectors said there was no warning of a potential problem at the mine site, although better training might have saved the final victim -- a paramedic who rushed to help a dying coworker without first putting on special breathing gear.
The mine, owned by Teck Cominco Ltd., had closed in 2001 after 92 years of operation and was in the process of being decommissioned. The unusual deaths were in a building used to sample water draining from the mine's dump.
The first victim was a contractor whose body was not discovered until two days later by a mine worker who called for help, but then died himself. The first paramedic to arrive thought she was responding to a drowning.
"On her way down the ladder she uttered an exclamation and questioned the presence of gas. By the time she asked that question it was too late for her to extricate herself," officials said. The second paramedic succumbed while rushing to her aid.
Officials said they still doing tests to see if they can recreate the conditions at led to the building filling up with deadly air, but have issued new safety rules for similar facilities at other mines in the province.