Story highlights "You could see the fireworks going off," says mother who watched from a nearby school

Everyone within a kilometer of the plant is evacuated

The plant is nearly an hour's drive southwest of Montreal

Two people died Thursday when a fireworks plant exploded near Montreal, federal police in Quebec said.

"You could see the fireworks going off," said Stacey Hylen, who was bicycling with her husband and 5-year-old daughter to a nearby school to watch her 10-year-old participate in a mini triathlon.

"The kids thought it was fireworks for the start of their triathlon," Hylen, 41, told CNN in a telephone interview.

The plant is located a mile north of the school in the town of Coteau-du-Lac, nearly an hour's drive southwest of Montreal.

As the explosions continued, it became clear that was not the case. "It was like a bomb," she said. "It was so loud. Like, at first you were thinking it was dynamite. It just kept going on and on. And then, when you saw the mushroom cloud it was like, woah! It just freaked me out. It shook. I had friends tell me it shook their house, and they live miles away."

Soon after, thick black smoke emerged from the roof of one of the factory's two buildings, both corrugated-metal structures about 20 feet tall.

The children were sent into the school gymnasium, she said. "The cloud was really black and scary looking. You didn't want to breathe it."

Fireworks shot from one building to the other, about half a football field away, and both structures were soon in flames, she said.

Across the parking lot, a store that sells fireworks and souvenirs was unscathed, she said.

Soon, police and fire vehicles arrived, but firefighters were initially unable to approach the buildings, which continued to be rocked by explosions.

Highway 20, which connects Montreal with Toronto, was shut in both directions for hours.

Residents and businesses within a kilometer of the BEM plant were evacuated as a precaution, Quebec Provincial Police Sgt. Joyce Kemp said.

An investigation was under way to determine the cause of the fire, she said.

The triathlon was canceled.