Fire officials continue to probe a house explosion in Kitchener.

It happened just before midnight Sunday at 429 Activa Avenue.

Assistant Platoon Chief Robert Gilmore tells 570 News four people were transported to hospital with undetermined injuries.

He says the early damage estimate is $500,000.

Gilmore says the cause is under investigation and the Fire Marshal’s Office will be contacted.

He expects firefighters to remain on the scene throughout Monday morning and probably into the midday trying to extinguish spot fires they have not been able to get to due to the collapse.

Video of fire recorded by Harry Scanlan on Twitter @footose