Firefighters are mopping up after a massive blaze at a South Vancouver sawmill that sent five workers to hospital early Friday morning.

The fire broke out between 5 a.m. and 6 a.m. at Mainland Sawmills, located at 8708 Yukon St. on the Fraser River, just east of the Oak Street Bridge.

"We ended up hearing a boom and somebody came running out of nowhere telling us, 'There's a fire. There's a fire.' Sure enough, we go over there and the whole back end's on fire," said Damien Carlsen, who was working in the mill.

"The whole place was just covered (in flames)—the whole basement floor."

Carlsen told CTV News he and six other workers were just 15 minutes away from finishing their shifts when the fire broke out.

Between 50 and 70 people were on the property at the time. All of them escaped, but some needed medical attention for smoke inhalation. One worker suffered a minor burn on the back of his head.

"They did walk to the ambulance, but they received medical attention from BC Ambulance and were taken to hospital,” Asst. Fire Chief Kevin Wilson said Friday morning.

All of the injured workers have since been released.

The fire was quickly upgraded to a third alarm. A raging tower of flames and thick, black smoke could be seen from far away as about 50 firefighters battled the blaze from all sides of the building.

"As I approached the south slope hill, we could see massive smoke coming across the sky and up the river," Wilson said.

Workers at the mill receive basic fire suppression training, and Carlsen said he helped firefighters for about 40 minutes before exiting the burning building.

It took crews more than three hours to get the fire under control.

Even then, there were concerns that part of the building could collapse and that the "thousands of gallons of hydraulic fuel and oil" at the scene could cause additional explosions.

Officials were also concerned about contaminants from the mill getting into the Fraser River. The coast guard and a hazmat team were on standby.

Tanks of oxygen and acetylene may have cause the rapid growth of the fire across the south side of the cavernous building, which contains multiple catwalks and platforms inside.

Firefighters say the blaze likely started the basement, but couldn't confirm how.

"Other than a worker using acetylene torches in the basement, we're not sure if it started from heating up a metal or what actually occurred," Wilson said.

The extent of the damage to the structure is unclear, leaving workers like Carlsen wondering if and when they'll be able to get back to work.

"We know there's enough damage inside that it won't be operating right away," said Alan Tjorhom, the mill's general manager.

"There's lots of water. Once we get all of that figured out, (we'll) have a good look and have a better idea on what we're going to do going forward."

With files from CTV Vancouver's Nafeesa Karim and Scott Hurst