However, the cause of the blaze remains a mystery.
Around 4:45 p.m. PT Friday, Nicole LeBrun, who lives across the street from the home on First Street and Rod & Gun Road, said she heard a loud boom.
'My window was open. All of a sudden I heard a bunch of of screaming and and some more bangs — loud, loud bangs — that sounded like someone got shot up," she said.
As the rear of the home went up in flames, neighbours said, they watched three men scramble out of the right side of the duplex and flee in a truck parked out in front — all of them badly burned.
Neighbour Jenni Wright said what she saw and heard was disturbing.
"I saw the flames, and then I heard the screaming. I saw the people come outside and get in the truck and rush off," she said.
Neighbour Bill Wright said he saw a big puff of something as he was coming down the road.
"As I got closer to our property here, saw a couple of pieces of clothing smoldering on the road,"he said. "I pulled over and looked at it and it was a couple of pieces of clothing that were still on fire a little bit."
Airlifted to hospital
Witnesses said one of the injured individuals drove himself and two others to the hospital. Two of them were taken by air ambulance to a hospital in Victoria, the other was airlifted to Vancouver.
All three are in critical condition, health officials said. One man suffered burns to 90 per cent of his body, one of the other two had burns to 70 per cent of his body.
Police investigating fire as 'suspicious'
Courtenay fire Chief Don Bardonnex said three people were in the building at the time of the explosion.
"On arrival, the whole backside of the building, from ground level to top, was in flames," said "The interior of the building in the back was in flames."
One of the backyard doors to the residence was blown off the hinges and there were several unidentified cylinders scattered across the lawn.
RCMP investigators are looking into the explosion as a suspicious fire.
"Every fire that is this serious and results in injuries is deemed suspicious until proven otherwise," says Comox Valley RCMP Const. Dean Biollo. "We're concerned obviously with the witness accounts of the explosions."
Police have applied for a search warrant to go through the home and expect to be on scene for a few days.
They say it will likely be Monday before they are able to update the investigation.