As investigators continue to work at an Alberta train derailment site Monday, a video uploaded online captures the moment when a controlled burn paves the way for a massive propane explosion.
The video, uploaded by the Transportation Safety Board of Canada, shows a controlled vent and burn of the propane that was being hauled by the derailed train.
Transport Canada's website says a vent and burn is the last resort when dealing with damaged liquefied petroleum gas (LPG) tanks. The process uses small, shaped explosive charges to blow small holes in the top of the car with simultaneous ignition of the escaping pressurized gas.
Story continues below the video
Thirteen cars, four carrying petroleum crude oil and nine carrying LPG, came off the tracks around 1 a.m. local time in the hamlet of Gainford, about 80 kilometres from Alberta's capital on Saturday.
Residents in the area were evacuated as firefighters battled the massive blaze.
A spokesman for CN Rail said the company had hoped a controlled burn on Sunday night would get rid of potentially explosive propane in some of the derailed tank cars.
"We had hoped the exercise would burn off all the propane in the cars, so that we could move in, remove the cars and allow people to go home. Unfortunately when we went to inspect the cars ... we found that not all the cars had vented their propane," Warren Chandler said.
"In the interests of safety, we withdrew," he said, adding that a controlled burn is a multi-step process and the cars have now been left to vent as the next steps are being assessed.
Here's a look at photos from the controlled burn and derailment:
Gainford Train Derailment
With files from The Canadian Press