A paramedic consoles a man after three people were overcome by an unknown substance and died at a mushroom farm in Langley, B.C., on Friday. (Darryl Dyck/Canadian Press)

The head of the umbrella labour organization in British Columbia is calling for an investigation into the deaths of three farm workers on Friday.

"We need a public inquiry that's going to find out how we stop these deaths," said Jim Sinclair, head of the B.C. Federation of Labour.

The three men died at Farmers' Fresh Mushrooms in Langley in circumstances that are still not clear.

Initial reports suggested they were mixing manure into compost when a pump broke at a nearby pumphouse. One of the men went into the pumphouse to fix the problem, and others followed when he was overcome with toxic gas.

But the regional manager for WorksafeBC, the provincial safety organization, warned against speculating about the cause of the deaths.

Gases are often produced on farms, Al Johnson said. "We can speculate at this point that it was the gas, we can speculate on what may have happened, but until the investigation is complete, we really don't know."

Three people are still in hospital after the incident.

One of the owners of Farmers' Fresh Mushrooms said she didn't know what happened. Her husband refused to comment after talking with police.

WorksafeBC and the B.C. Coroners Service are investigating the accident.

The labour federation has been pushing for better conditions and safety protection among the mostly immigrant, non-English-speaking B.C. farm workers.

In March 2007, three women were killed when a van carrying 17 workers rolled over on Highway 1 near Abbotsford in B.C.'s Fraser Valley.