A powerful explosion at a Mississauga warehouse came without warning on Wednesday morning, injuring a group of firefighters who were trying to make their way inside.

It was just after 4:30 a.m. ET when firefighters were alerted to the fire at an industrial warehouse located at 2797 Thamesgate Dr., just north of Toronto Pearson International Airport.

As the first firefighters on the scene attempted to enter the building, a large explosion caused a wall to collapse, briefly trapping them inside.

Mississauga’s deputy fire chief, Greg Laing, told reporters that the responding fire crews were not aware that the building contained explosive materials.

"Normally, fires progress in a certain way and that one did not," Laing said Wednesday afternoon.

"That one went from a building and contents fire emitting smoke, to an instantaneous destruction of the entire building, which we normally only associate with an explosive device that would do something like that."

Peel Regional Police Const. Thomas Ruttan said Wednesday that the warehouse is a storage facility for imported goods that included butane lighters and aerosol cans of insect spray, which appear to be the cause of the explosion.

Four firefighters were injured. At least three of those firefighters were taken to hospital. One was treated for smoke inhalation.

The explosion also blew out the windows of a fire truck. The heat melted parts of its exterior.

"We are incredibly fortunate that nobody was killed," Laing said.

Damaged fire truck

The explosion at the warehouse knocked out the windows of this Mississauga fire truck. Parts of the vehicle also melted. (Mike Heenan/CBC)

The most seriously injured firefighter was reported to be in critical condition when initially taken to hospital.

Fire officials offered conflicting information on his status as of late Wednesday afternoon — Laing said he had been informed that individual was in serious, stable condition, while Fire Chief John McDougall said he remained critical.

"They are improving, there’s two or three of them that are undergoing surgery and some need more surgery but they seem to be stable," McDougall told reporters.

"The most serious one is reported to me as being critical, but stable. So, we’re hoping that improves over the next few hours."

Those firefighters suffered "blast injuries," as opposed to burns, McDougall said.

Mississauga Fire

Police said air quality was a major concern following the Mississauga, Ont., fire due to the potential nature of the substances stored in the building. Air quality tests were conducted throughout the day, but no neighbourhood homes had to be evacuated. (Tony Smyth/CBC)

McDougall said the injured firefighters all are "very experienced" and each had between five and 20 years of service.

Mississauga Mayor Hazel McCallion said the city has dealt with many fires over the years, though one like this was not expected.

"It really has been a very sad day for us," she said, when speaking with reporters Wednesday afternoon.

McCallion said it is her understanding that the building was inspected by firefighters a year ago, but it was vacant at the time of that inspection.

The mayor said city staff are trying to confirm whether or not the business in that building had been granted an occupancy permit.

Ken Cree, the former manager of the property, said the 2,800-square-metre warehouse was sold last year.

Mississauga Fire

The fire at a Mississauga, Ont., warehouse Wednesday morning has been extinguished, but black smoke continues to billow from the building's remains. (Linda Ward/CBC)

"The building is fire-monitored and the building is sprinklered, so I’m not quite sure why it got so bad," he told CBC News.

Twelve hours after they first arrived, firefighters were still dealing with hot spots at the scene.

The cause of the fire remains unknown.

The Office of the Fire Marshal, the Ministry of Environment and the Ministry of Labour were at the scene, in addition to investigators with the fire service.

The CBC’s Shannon Martin reported that the Ministry of Environment had a mobile unit on scene. Air quality tests were conducted throughout the day, but no neighbourhood homes had to be evacuated.

The Mississauga Fire Fighters Association tweeted their thanks for the thoughts and prayers of others on Wednesday.

Firefighters from other cities tweeted their condolences to their colleagues in Mississauga:

With reports from the CBC's Shannon Martin and The Canadian Press