Five people are injured, including one critically, after an industrial explosion in Sarnia, Ont., police said Saturday.

The explosion occurred around 2:40 p.m. ET at Veolia ES Canada Industrial Services Inc. on Scott Road north of Churchill Road/Highway 40, according to Sarnia Police Services.

When firefighters arrived on scene, they extinguished the fire over a span of two hours and dealt with injured people.

Eight people were working at the facility when the explosion happened, said Sarnia Police Staff Sgt. David Bonnema.

Five people were transported to a local hospital. One individual with critical injuries was later airlifted to another hospital.

Initial reports indicated the person was taken to a London, Ont., hospital. Bonnema said the patient was taken to a burn unit at a Hamilton hospital.

Sarnia police and Ministry of Labour investigators are conducting an initial investigation into the cause of the explosion, which is currently unknown. (Andreanne Baribeau/Radio-Canada) Three others suffered serious injuries, while one other person has minor injuries, Mayor Mike Bradley said in an interview with CBC News. He had not had an update on the condition of the five injured people since late Saturday afternoon.

Three others were assessed on site and released, said Jeff Brooks, a spokesman with local Lambton County paramedics, to the Canadian Press.

James MacDonald, a spokesman for air ambulance Ornge, told the Canadian Press that an Ornge helicopter had been dispatched from London and was on standby at a Sarnia airport.

"It has not yet been determined whether we are required to transport a patient to another centre, but we are standing by in case we are needed," he said.

No known cause

It is still unclear what caused the explosion, which partially collapsed a building's roof and some of one exterior wall.

Nearby residents reported hearing at least one big bang in a discussion on a Facebook page for alerts in the area. Bonnema said the explosion could be heard up to one kilometre away and damage to the building is extensive.

Sarnia police and Ministry of Labour investigators are conducting an initial investigation into the cause. The Ministry of Environment will also be involved in the investigation, said Bradley.

Fire crews were on site throughout the night to periodically check for hot spots, Bonnema said, and investigators will likely begin digging into the rubble on Sunday.

'It's not an easy job'

Veolia Environmental Services's Sarnia location offers industrial cleaning and maintenance services, according to its website.

It's a small company that works in tank maintenance, said Bradley.

The company's industrial cleaning and maintenance services include chemical cleaning, emergency response for spill containment and clean up, hydroblasting, tank cleaning and separations, and vacuum services.

"It's not an easy job to do ... and safety is always a factor," he said.

The company is located in an industrial area free of residential homes. There is no impact from the explosion to other industries near the scene.

Officials are not concerned about airborne chemicals, said Bonnema, as the incident was strictly a structure fire.