The drug unit of the Barrie Police are investigating a house fire that sent 11 people to hospital — four of them with serious injuries — in Barrie, Ont., on Friday evening.

Police suspect "drug-related activity" at 59 Collete Cres., in the city north of Toronto, ​caused an explosion and fire around 9 p.m. ET.

"We believe the explosion may have been caused by drug-related activity in the garage," Barrie Police spokewoman Const. Sarah Bamford told reporters on Saturday afternoon.

Investigators suspect butane canisters being used to produce cannabis resin caused the explosion.

Barrie Police are investigating, together with the Ontario Fire Marshal.

Investigators say they are in the process of obtaining a search warrant, which they plan to execute on Sunday. No charges have been laid.

Deputy Fire Chief Jeff Weber said fighting the blaze was a challenge.

"They had very difficult conditions in the upstairs when they went to the second floor to do a primary search, and once they confirmed that they had all the people accounted for that were in the house, they retreated back outside to a defensive attack," Weber said.

Neighbours said they heard explosions, or what sounded like loud bangs, during the fire, which was brought under control by midnight.

Several children inside

Three men and one woman who were in the garage at the time were taken to a Toronto-area hospital with serious injuries, Bamford said. Two were still in critical condition by late Saturday afternoon. The other two were in stable condition.

A 15-year-old girl — the oldest of several children on the property — also suffered minor injuries.

Police say there were six adults and eight children in total at 59 Collete at the time of the fire; the youngest being three weeks old. Some were visiting the house, while others lived there full- or part-time, police said.

Police have described the residents as a blended family. The children are now with family members, Bamford said, and the Children's Aid Society has been made aware of the situation.