Authorities are still digging through the wreckage of a Nanaimo heritage building that was destroyed by fire, but it could be days before they find out a cause.

Four fire investigators spent the day at the old Jean Burns building on Commercial Street with an insurance company and the building’s owner, according to Nanaimo Fire Chief Craig Richardson.

They have yet to figure out what sparked the blaze that levelled 15 businesses and suites after flames broke out Thursday evening and will be investigating “for a number of days yet,” Richardson said.

But the on-scene investigation is only one part of a sweeping probe that will see officials interview business owners and bystanders.

“There were a number of people affected by this incident given the number of people and businesses and witnesses that might’ve been there,” said Richardson.

He said crews had a tough time battling the blaze because of the age of the building.

“The construction type is very difficult for us to manage with fighting fire, given the number of void spaces, multiple ceiling levels and hidden ceilings and attic spaces…no sprinklers and things of that nature,” he said.

Richardson also explained that a video showing an incredible explosion blowing out windows of the building wasn't actually a backdraft, as many described it.

"What actually occurred there was what we would call a smoke explosion," he said. "It ignites when it reaches critical temperatures, and when it ignites it creates a pressure that causes the windows to blow out and it's quite dramatic and spectacular."

While many are mourning the loss of the building, an iconic part of the city’s downtown core, others are voicing their support for the fire crews that tried to save it.

“The responders are also being recognized by citizens and receiving emails and telephone calls thanking us for the work we did,” Richardson said. “But we don’t want that to detract from the fact that the owners of these businesses have suffered some significant losses.”

No one was injured in the fire.

The building is expected to be a total loss.