A series of explosions subsequently leapt from the charred building after the initial blast at B.E.M. Fireworks Thursday morning near Valleyfield, Quebec. Images from the scene showed a building near a major highway completely destroyed.
Provincial police said two bodies were found in the wreckage but they did not identify them.
Nearly two hours after the blast, fireworks could still be heard exploding at the scene of the fire that continued to burn out of control hours after the explosion, according to witnesses.
"All of a sudden I heard, 'boom, boom, bang,'" Mario Cramerstetter said from his office, which is just over a mile (1.6 kilometers) away from the site.
"Initially I thought it was a bulldozer or a tractor but then we saw a big puff smoke. We kept on hearing popping, fireworks going off all the time."
Cramerstetter said it looked a small building caught fire first and then spread to a larger storage unit.
"We got really, really, really scared," said Ginette Liboiron, who runs a convenience store across a 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," she said. "It went high up in the air, then it became black, black, black."
Police ordered the surrounding community of Coteau-du-Lac evacuated. A nearby highway was also closed in both directions.
Police also said low traces of metal materials were found in the surrounding area.
Video captured by a news helicopter showed fireworks igniting inside the remnants of the smoldering building.
According to its website, B.E.M. has been designing and manufacturing pyrotechnics and fireworks for 25 years.