Investigators are still trying to understand what caused an explosion at an industrial plant in Sherbrooke, Que. on Thursday that killed two employees and seriously injured several others.

Nineteen people were taken to hospital in the blast in the Eastern Townships town, including five with serious burns. Four burn victims were transferred to a Montreal hospital.

A doctor from the Hotel Dieu burn unit said three of the victims experienced burns to 12 to 35 per cent of their bodies, while one man suffered burns to 90 per cent of his body.

He remains in critical but stable condition.

Fire crews later pulled out a woman who was hiding among the debris, terrified but alive.

A Sherbrooke police spokesperson said finding the woman alive and well was a miracle.

“She was hiding under a desk,” Martin Carrier told CTV Montreal. “It was a miracle she was intact with no physical injuries.”

The explosion happened at a Neptune Technologies & Bioresources facility, which produces and exports health products derived from marine life, such as Omega-3 supplements.

The explosion site Friday morning is being described as a scene of devastation. The only walls that remain standing have been scorched black by the fire that followed the explosions. Across the town, the city has lowered flags to half-mast.

Neptune confirms that the two who were killed were employees. The company is promising to co-operate with police during the investigation and has also brought in a psychiatric support team to help traumatized workers.

"We're in terrible shock over what's happened," Michel Chartrand, chief of operations for the company, told reporters at a press conference Thursday evening.

“We want to understand what happened.”

Witnesses say there was at least one large explosion followed by a series of smaller ones. The blast triggered a fire that sent thick dark smoke billowing in the air for several hours before a crew of 50 firefighters could get it under control.

Following the explosion, a toxic cloud of black smoke could be seen from kilometres away.

The fire was fuelled in part by 15,000 litres of acetone at the plant, which is a highly flammable substance.

There were concerns about toxicity because acetone can cause irritation if ingested. But tests revealed the smoke did not contain toxic chemicals. As well, the wind kept the smoke away from residential areas.

Tanks of acetone remain on the explosion site and investigators are waiting for the chemical to be removed before surveying the damage. It’s expected that the acetone will be removed Saturday.

The initial blast was so violent, it was heard across the city and set a local record for 911 calls, according to local fire chief Gaetan Drouin.

"Just looking at the damage to the building you can see it was probably very violent," Drouin said.

Police say more than 100 people in Sherbrooke and surrounding suburbs phoned within a minute of the blast.

Sherbrooke Mayor Bernard Sévigny said the explosion was a tragedy not only for the employees at the plant, but for the entire community.  

With a report from CTV Montreal’s Tania Krywiak