The subsequent fire then prompted a shutdown of the adjacent highway, causing a huge traffic jam, and an evacuation of dozens of homes, businesses, a campground, and a school.
Provincial police said the bodies of two female employees were found in the wreckage.
The initial explosion at B.E.M. Fireworks occurred just before 9 a.m. Thursday and completely destroyed the building, which was reduced to charred scraps.
A series of sparkly blasts followed.
Those multicoloured clusters popped above the rest of the thick, dark smoke emanating from the blast site, located near Valleyfield, Que., roughly 60 kilometres southwest of Montreal.
The original blast vibrations could be felt for kilometres and the smoke was also visible from neighbouring municipalities.
About 150 firefighters, from 14 different departments, were called in.
"We got really, really, really scared," said Ginette Liboiron, who runs a convenience store across the highway overpass.
"I thought my store was falling to the ground. It shook like you can't imagine... We all went outside to see and saw the big, incredible smoke.
"Then the fireworks went off."
According to its website, B.E.M. has been designing and manufacturing pyrotechnics and fireworks for 25 years.
One area resident said the explosion scared her cats. Another said his dog slammed into a wall.
The two panicked felines went scampering under the bed. Madeleine Boucher said she watched the explosion from her window 700 metres away.
"It was impressive," she said, joking that the incident would have prettier at night.
"Wasn't very pleasant, though... I wouldn't say I was scared for myself, no, but I'd have been scared if I lived closer."
Another man, whose dog slammed into the wall, was about to have breakfast with his wife.
Roland Desforges says his wife thought the explosion might have been a plane crash or an act of war. But he says he instantly suspected it might be the neighbouring fireworks plant.
He says he went to the overpass to get a look.
He believes the initial explosion occurred in the middle of three buildings on the compound: in the production plant, which he says is centred between the fireworks store and its storage facility.
"In seconds there was nothing left," Desforges, 68, said of the plant.
The company occupies a sprawling property near Highway 20 that includes a store and warehouse. According to the Quebec business registry, the company employs between six and 10 people.
An investigation into the blast cause is underway. But the provincial government has already expressed some concerns about a lack of fire hydrants near the site.
Fire crews struggled to access water as the closest hydrants were on the other side of the highway. The town's mayor, Robert Sauve, justified that absence of hydrants next to a pyrotechnics plant by saying it had been there for decades.
The area evacuees were allowed to return home in the afternoon.
Liboiron said she has run the nearby store for 37 years and had never worried about being next to the fireworks factory.
That indifference went up in a monumental puff of smoke.
"It was gigantic," she said.
"It went high up in the air, then it became black, black, black."
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