Three of four injured firefighters remain hospitalized on Thursday a day after a fire and explosion leveled an industrial building in Mississauga.

Mississauga fire chief John McDougall said Thursday from the location of the fire on Thamesgate Drive that Tim Casarin, 46, Capt. Brad Hamilton, 38, and acting Capt. Al Mills, 47, suffered “blast-related injuries” including broken bones. Daryl Roy, 35, was hospitalized and treated for smoke inhalation but was released Wednesday.

“Two of them have had to have surgery and we understand the third firefighter may be released shortly,” he said.

The fire began at a storage facility at 2797 Thamesgate near Airport Road around 4:30 a.m. on Wednesday. Three of the firefighters were injured when a blast caused a wall to collapse.

Fire investigators were at the scene Thursday trying to figure out what exactly was in the building. They have been told there were butane tanks, bug spray, pesticides and printer toner inside, some of which exploded during the fire.

Mississauga deputy fire chief Greg Laing said the business had not yet been inspected to determine whether it met with proper building and fire codes. The business may not have had an occupancy permit, and may be why crews didn’t know what was inside.

McDougall said had firefighters known what was inside “they certainly would have approached it differently.”

The Ontario Fire Marshal’s office is leading the investigation over the next few days.

“We have heavy equipment in the background being supplied by the municipality to try to find clues and evidence with regard to the potential cause of the fire and explosion,” investigation supervisor Mike Potvin told reporters.

CORRECTION: An earlier version of this story mistakenly reported the number of firefighters still in hospital.