Two paramedics responding to an emergency call at a decommissioned mine in British Columbia were among four people who died there.

The tragedy at the decommissioned Sullivan Mine began when a contractor who had been testing water in a pumping station was reported missing. A second worker found him dead and called 911 but was also overcome.

Two responding paramedics met the same fate. It isn’t yet known whether hydrogen sulfide gas, carbon monoxide, insufficient oxygen levels or something else caused the deaths. The responding paramedics were not wearing protective equipment when they entered the pumping station.

The Teck Cominco lead, silver and zinc mine, decommissioned almost five years ago, once employed 2,000 workers.