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The explosion happened about 10 a.m. Thursday at the refinery owned by Calgary, Alberta-based Husky Energy, prompting authorities to evacuate a 3-mile (5-kilometer) radius around the plant, as well as a sparsely populated 10-mile (16-kilometer) corridor south of it where the plume of smoke extended for miles.

At least 10 people - one seriously injured - were taken to area hospitals, said a spokeswoman for Essentia Health-St. Superior Fire officials said the fire was out by noon after reports of at least five people rushed to the hospital.

The area was said to be contained and controlled after an explosion at Husky Energy oil refinery in Superior, Wis. Mary's Medical Center, which operates hospitals in Superior and nearby Duluth, Minnesota.

There was an explosion Thursday at an oil refinery in Wisconsin that resulted in "multiple casualties", a local Fox affiliate reported. "When you see that kind of smoke, it means you're getting a lot of unburned hydrocarbons".

The fire chief says there are no known fatalities.

Asphalt is a petroleum product that when burned emits chemicals in gaseous form and small particles that can linger long after the smoke dissipates, said Wilma Subra, a chemist with the Louisiana Environmental Action Network who has examined past refinery accidents.

Talarico says another five are being treated at St. Mary's Hospital in Superior, Wisconsin, where the explosion happened.

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It isn't clear how many people the evacuation order will effect. She said the extent of injuries is unknown. She doesn't know the condition of that person.

Superior School District cancelled classes Friday after the city declared a state of emergency and police officers were seen going door-to-door in an attempt to evacuate residents.

A contractor who was inside the building told WDIO television that the explosion sounded like "a sonic boom" that happened when crews were working on shutting the plant down for repairs.

Superior police are advising people to stay away from the area and roads around the refinery have been blocked off. It produces asphalt, gasoline, diesel and heavy fuel oils, largely using heavy crude oil imported from Canada.

Husky Energy of Calgary, Alberta, bought the refinery past year for $492 million.

Several people have been injured in an explosion at a refinery in Wisconsin. U.S. Department of Labor spokesman Scott Allen said the Occupational Safety and Health Administration fined Calumet Superior LLC $21,000 over emergency response and flammable liquids violations in 2015.

The refinery, which can process up to 38,000 barrels of oil a day, had additional workers on site preparing for a plant-wide overhaul, he said.