Investigators are at the site of Thursday's fire at the CNRL Horizon project near Fort McMurray. (Submitted)

Two Alberta Occupational Health and Safety officers are trying to find the cause of an explosion at the Horizon oilsands site and whether there were any safety violations.

A stop-work order has been issued for the site in northeastern Alberta where the blast sent flames and smoke hundreds of metres into the air Thursday afternoon. Five employees or contractors were hurt, site owner Canadian Natural Resources Ltd. said in a release Friday.

Three workers were hurt directly by the explosion — one had second-degree and third-degree burns, another had first-degree burns, while a third was treated for shock.

Two other workers were hurt around the same time — one with back injuries and another with leg injuries.

Worker in Edmonton hospital

Only one person remains in hospital. That worker was moved to Edmonton.

Employees were on the job Friday, but most were sitting around talking and playing cards, a CNRL worker told CBC News. CBC News is not naming the man for fear he will be blacklisted for other jobs in the oilpatch.

“Everybody’s trying to crack jokes, trying to ease each other,” he said. “Everyone’s stressed out. Everyone knows how lucky everyone was.”

The worker was near the coker when it exploded at about 3:30 p.m. MT, he said. A coker uses heat to convert bitumen into crude oil.

“It was the biggest bang,” he said. “You could feel it in your bones. It was a chilling feeling.”

The fire was put out by 7:15 p.m.

The structure is still smouldering and flagged off because there is the danger the coker might collapse, he said.

CNRL's senior vice-president of operations for Horizon, Peter Janson, said the company is also investigating the cause of the incident.

"The objective at the end of the day, very clearly, is to understand what happened, why and to prevent it from reoccurring," Janson said.

Alberta Employment and Immigration Minister Thomas Lukaszuk said the province will be hiring a third-party engineering consultant to look into the explosion.

Facing charges

Canadian Natural Resources is already facing charges under Alberta's Occupational Health and Safety Act from an earlier incident — two foreign workers from China died when a tank they were working on collapsed in April 2007. That case is ongoing.

In 2008, another worker died when the floating backhoe he was operating tipped into a tailings pond.

Lukaszuk said there was no evidence the Horizon site is any more dangerous than any other oilsands facility.

"This site cannot be isolated as a more dangerous site to work," he said. "That simply cannot be substantiated by the number of accidents."

NDP labour critic Rachel Notley blasted the government for failing to enforce safety standards in the oil industry and called on the province to make the findings of the investigation public.

"I don't believe that Albertans can trust this government to keep workers safe and the government needs to do better and they need to make everything public so that we can start to develop that trust," she said.

The president of the Alberta Federation of Labour, Gil McGowan, called the explosion "massive and unprecedented" and troubling for those in the labour movement.

"We've seen accidents on oilsands sites before, and we've seen coker fires before, but to the best of our knowledge, never before has a coker completely blown apart as the Horizon coker did yesterday," McGowan said.

McGowan is urging Lukaszuk to give the investigation as high of a priority as he can, and dedicate all the resources needed to make it as thorough, timely and as transparent as possible.

He is also calling on the province to set up an expert panel to examine the pace of oilsands development in Alberta and how it is affecting worker and public safety.