SAINT JOHN, N.B. -- A week after a fiery explosion at Canada's largest oil refinery rocked the east side of Saint John, N.B., investigators say they have yet to examine the blast site because it remains a "hot zone."

At least four workers received minor injuries on Oct. 8 as swirling flames and black smoke rose into the sky above the sprawling Irving Oil refinery, which produces more than 320,000 barrels of refined products every day.

Workers hit by the explosion reported hearing a loud hissing sound from a diesel treating unit before they were engulfed in a wall of fire.

WorkSafe NB, the Crown corporation that oversees the province's Occupational Health and Safety Act, was called in to determine the cause of the blast.

"We haven't been able to get right to the site yet," an agency spokeswoman said Monday. "It's still a hot zone."

She said it remains unclear when the site will be safe to examine.

"From my understanding, as it's a refinery, there's chemical reactions at play," she said, adding that witness interviews were still being conducted.

"They're still spraying foam from a distance ... We're gathering all of the information that we normally would in the meantime."

At the time of the explosion, there were as many as 3,000 workers at the refinery, many of them contractors performing maintenance. About 1,400 people typically work at the 315-hectare site.

Irving Oil did not respond to a request for an interview.

Mayor Don Darling has said the city's large industrial base comes with risks and there needs to be a broader discussion about the interaction between residents and industry.

Darling has said he's satisfied with the city's response to the explosion, but he said there will be a review of the city's actions, including its communication strategy.

It was the fourth serious explosion at the refinery in the past 20 years.

A blast in June 1998 killed one worker and injured two others.

In March 2002, three full-time contractors were sent to hospital to be treated for burns following an explosion at the top of a 10-metre stack.

And in September 2012, a 1,100-barrel tank of potassium hydroxide blew up, leaving one worker with minor injuries. Irving Oil said the tank was undergoing scheduled maintenance when it became "over-pressurized." But it did not catch fire or leak.

In January of this year, Irving Oil apologized for a butane leak that forced the evacuation of businesses on at least two streets in Saint John. Police say the rupture happened in a line that runs from the Canaport facility on Bayside Drive to the Irving Oil refinery.

The refinery, which is flanked by several neighbourhoods, opened in 1960. More than half of its production is exported to the northeastern United States.