Investigators have determined the cause of a $4.5 million-fire that ripped through a downtown apartment building last week.

Officials say the fire at Royal Scot Apartment located near 92 St. and 104 Ave. was started by a propane-fuelled heater igniting combustible material. The heater was being used as a method of exterminating an infestation of bed bugs within a fourth-floor suite.

"We're looking at the equipment. Was there failure in the equipment? We're going to be looking at whatever the procedures were for the particular process they were using here and whether the operator had follow those procedures or not," said chief of investigations, Wayne Pawlowski.

Four people were seriously injured in the fire, including two children.

Residents were getting assistance from the Red Cross.