A mother, her daughter and son are among four people who died in a fire that tore through a multi-resident dwelling in Oshawa, Ont., early Monday in what officials are now calling "one of the worst events" the city has experienced in years.

The blaze broke out shortly after 8 a.m. ET at the house on Centre Street North and Colborne Street West, near Bond and Simcoe streets, Oshawa Fire Services said.

Officials say investigation will take days to determine cause 1:10

When crews arrived on scene, Chief Derrick Clark said they encountered "heavy fire and heavy smoke conditions" and a snowstorm descending on the area.

The winter weather made for "very difficult conditions, both for crews driving en route here, as well as fighting this fire," Clark added.

'Severe fire conditions'

Fire crews initially started removing multiple people who were trapped inside the two-storey residence, but were forced to vacate the building due to "severe fire conditions that worsened."

Two adults and two children are dead following a fire at a rooming house in Oshawa, Ont., that Mayor John Henry says is 'one of the worst events' the city has experienced in a long time. (CBC)

"Firefighters made several heroic attempts to get in and to get to the upper levels," Clark told reporters Monday afternoon.

"They fought their way up through intense heat and smoke conditions. Even as fire was around them at all times."

Crews extinguished the flames from outside the building before searching for the remaining residents, Clark explained. He estimates around 11 people lived in the two-storey home.

3 victims remain inside home

Lindsey Bonchek, 36, and her daughter, Madeline, 9, were trapped inside the house, and believed to be dead, said Durham Regional Police. Bonchek's four-year-old son, Jackson, was pulled from inside the home and rushed to hospital where he later died of his injuries. His cause of death has not yet been released.

Steven Macdonald, 50, was also trapped inside the home and is believed to be dead. His relationship to the other victims is still unclear. A GoFundMe page has been set up to help cover the cost of his funeral.

"This is a very tragic event," said Clark.

Firefighters are working to remove their bodies on the upper and lower levels of the home so they can be taken to the coroner's office for a post-mortem evaluation that will determine their cause of death.

Steven Macdonald, 50, shown here on the far left, ran into the burning Oshawa, Ont., house in an attempt to save those trapped inside, according to a neighbour. (Facebook)

"I'd like to extend our thoughts and prayers to the families involved," said Oshawa Mayor John Henry.

"It's always nasty when there's a fire and it's always upsetting when there's a loss of life. To the families, we pass on our deepest sympathies."

Three other residents were taken to hospital where they were treated for smoke inhalation and cuts from broken glass, according to Clark. There is no update on their current conditions.

"We've had some major industrial fires over the years, but in my recollection, this would be one of the worst events we've had in a long time," Henry said.

Fire marshal investigates

Ontario Fire Marshal Ross Nichols and two investigators from the Office of the Ontario Fire Marshal will probe the cause of the fire. The provincial agency is called on to investigate any fire that results in fatality, serious injuries or involves over $500,000 worth of loss.

Clark anticipates crews will remain on scene for up to five days.

The two-storey home was gutted by the blaze. (Grant Linton/CBC)

It's unclear where the blaze started and if there were working smoke alarms, Clark said. He noted the rear of the house seemed to suffer the most extensive damage.

"I think the message here is that no matter what dwelling you live in, if you have working smoke alarms that's the key," he said.

Initial reports also indicated that an explosion had occurred, but officials have yet to confirm this.

Witnesses told CBC Toronto they heard a sharp bang before the fire broke out.

"We originally thought it was a car accident," said William McCarthy who lives next door.

Witnesses say the fire spread rapidly, consuming the rear of the house. (Viewer video)

"We came out and saw the flames ... it instantly took up the back corner of the house. It took maybe five seconds."

Another neighbour, Laura Green, described the blaze as "horrific" and said, "It breaks my heart, especially for the kids."

Green claims she saw residents running from the home.

"You could see the flames behind them, you could hear them yelling, 'Get out, get out,'" she said.