Woodstock fire officials are continuing to investigate a Friday house fire that sent four people to hospital.

Heavy equipment has been brought in to help investigators sift through scene, where a fire engulfed a house at 598 Alberta Ave. around 4:45 p.m. Friday.

Fire officials are hoping the equipment will help them determine a cause and origin of the blaze. The Ontario Fire Marshal's office is also helping with the investigation.

"911 callers had heard an explosion in the residence," fire chief Scott Tegler said.

Officials say three people remain in stable condition in hospital with burns.

The neighbouring residences at 602 and 594 Alberta Ave. sustained heat damage to their exteriors as well, police said.

The fire chief said fighting the fire in the frigid cold was a challenge.

"The gear freezes up. It's probably one of the most difficult challenging jobs you can do in this type of weather so we've rotated crews in ...to thaw out their equipment."

Constable Steve McEwen tells CTV News it may be several weeks before they know what happened.

"You know this whole investigation with all the water and all the ice, it takes time to get through that and come up with any idea what has happened," says McEwen.