A day after a massive fire destroyed a Mississauga warehouse, investigators are picking through piles of charred, twisted metal and bricks, trying to determine what started the blaze.

Fire crews were still at the scene on Thursday putting out remaining hot spots, after the fire ripped through the Thamesgate Drive facility near Toronto’s Pearson International Airport early Wednesday.

Paramedics were also called to the scene Wednesday after four firefighters were injured while fighting the three-alarm blaze.

Brad Hamilton, Al Mills and Tim Casarin, are now recovering from broken bones and internal injuries, and may soon be released from hospital. A fourth firefighter, Daryl Roy, was released from hospital Wednesday afternoon after being treated for smoke inhalation.

“Two of them have had to have surgery, and we understand the third firefighter may be released shortly,” Mississauga Fire Chief John McDougall told reporters Thursday.

Investigators still don’t know what caused the fire, but were told there were butane tanks, aerosol cans, printer toner and pesticides in the building.

Mississauga Fire Deputy Chief Kevin Duffy said the blaze escalated quickly, and that, soon after fire crews arrived, an explosion caused part of the building to collapse.

Investigators removed a hard drive from the security camera inside the warehouse on Thursday morning.

Mississauga Mayor Hazel McCallion told reporters that the owners of the warehouse had not filed an occupancy permit for the building.

She said the building was vacant the last time it was inspected, which is why firefighters had no idea what they were up against when they arrived at the scene.

“We don’t know of any occupancy permit that was granted by the city for the building to be occupied with the type of material,” she said on Wednesday. “The explosions caused the building to collapse and therefore seriously injured our firefighters.”

The Fire Marshal’s office and the Ministry of Environment are investigating.

With files from The Canadian Press and CTV Toronto’s John Musselman