Three firefighters were injured - one critically - when an explosion occurred inside a burning warehouse in Mississauga and a fourth suffered smoke inhalation while battling the inferno early Wednesday morning, a senior official says.

Mississauga Fire Deputy Chief Greg Laing said the blast and a partial building collapse occurred as the first crew arrived on scene shortly before 5 a.m.

“They were making entry when the building exploded," Laing told CP24 reporter Cam Woolley at the scene on Thamesgate Drive, near Airport Road and Steeles Avenue. "The explosion pretty much demolished the building."

The injured firefighters escaped the crumbling structure and were transported to hospital by paramedics. One of the injured crew members - a man in his 40s who has been on the job for more than 10 years - was listed in critical condition on Wednesday afternoon but fire chief John McDougall said his condition has stablized and he is expected to survive.

Two other firefighters remain in hospital. One firefighter was kept for observation at St. Michael's Hospital overnight Wednesday and the other is being treated at Etobicoke General Hosptial for various injuries including a broken femur, foot and pelvis. 

While firefighters gathered at the hospitals where the injured men were receiving care, politicians and fellow emergency workers reached out to offer their support to the fire department.

“Our thoughts and prayers are with the firefighters and their families," said Mississauga Mayor Hazel McCallion in a news release. "We are grateful for all emergency services personnel who are on site."

At a news conference on Wednesday afternoon, McCallion said the building was vacant the last time it  was inspected by firefighters a year ago and an occupancy permit for the building has not been found.

Building packed with lighters, bug spray

Peel Regional Police spokesman Const. Thomas Ruttan said the blaze occurred at a storage facility for imported goods, and butane lighters and aerosol cans of bug spray were exploding at the height of the fire.

Dozens of canisters were scattered on the ground outside the building, which has been occupied by G.E. Logistics/G.E. Forwarders for about a year.

A wall of flames could be seen from afar and a huge cloud of black smoke was swept to the south by strong winds.

Ruttan assured residents the smoke is no more toxic than usual, but he encouraged them to remain indoors and keep their windows and doors closed.

As the fire burned out of control, witnesses heard loud explosions and saw fireballs shooting into the air.

Residents awakened by 'popping noises'

Bianca Genova said she was awakened by “popping noises” and looked out her window to see the inferno across the street.

“We can actually feel the heat on us as we open the window,” Genova told CP24 anchor Lindsey Deluce by telephone. “It was extremely hot and quite terrifying.”

“I thought there was a train accident,” said James Ross, who was awakened by the noise. “It sounded like trains going over a track. I came outside and there was a big ball of black smoke and it was just red all over the sky.”

Arnim Cerella could see the flames and smoke from his sixth-floor condo unit about two kilometres away.

“We can see the flames going higher than the roof,” Cerella told CP24. “The smoke is just pitch black.”

Firefighters eventually gained the upper hand, as they poured water on the flames from a safe distance. Once the fire was contained, they focused on putting out hot spots.

The cause of the fire remains under investigation.

A hazardous materials team has been called in to assess the burned-out building and its contents.

Fire disrupts GO services, school buses

Roads surrounding the building are sealed off and police are urging motorists to stay away from the area.

Road and rail closures affected GO Transit buses on the 34 Brampton via Airport Service route, school buses and trains on GO's the Kitchener line.

Train service was suspended between Bramalea and Union stations.

The warehouse is just north of Pearson International Airport, but the fire is not disrupting flights.

With files from CP24 reporters Cam Woolley and Cristina Tenaglia.

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