SUPERIOR, Wis. >> An explosion rocked a refinery in northwestern Wisconsin today, injuring at least 11 people, forcing the evacuation of homes, schools and a hospital, and sending a plume of noxious smoke into the air.

Authorities said a tank containing crude oil or asphalt exploded at the Husky Energy oil refinery in Superior, a city of about 27,000 that borders Minnesota and the westernmost tip of Lake Superior. That prompted them to order the evacuation of a three-mile radius around the refinery, as well as a 10- mile corridor south of it where the smoke was heading.

It was unclear how many people were being evacuated. The refinery is in an industrial area, but there’s a residential neighborhood within a mile to the northeast. The corridor downwind to the south of the refinery is sparsely populated.

There were no reported deaths, but at least 11 people were injured, including one person who was seriously hurt. They were being treated at hospitals in Superior and nearby Duluth, Minnesota.

Fire officials said the explosion happened around 10 a.m. and the fire was extinguished by 11:20 a.m., though the plant was still smoking. Superior police later tweeted that the fire had reignited and urged residents living within the evacuation area to leave. Police blocked roads into the area around the refinery. Three schools and St. Mary’s Hospital in Superior were being evacuated as a precaution.

Essentia Health spokeswoman Maureen Talarico said five people injured in the explosion are being treated at St. Mary’s Medical Center, a Level II trauma center in Duluth. She said emergency room physicians described those patients as awake and alert. Another five are being treated at St. Mary’s Hospital in Superior, Talarico said. One person suffered a serious blast injury while the other nine had non-life-threatening injuries.

In Duluth, St. Luke’s Hospital was treating one person who was in fair condition and did not expect to receive any others, spokeswoman Jessica Stauber said.

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

No damage estimate was available.

Calgary, Alberta-based Husky Energy bought the refinery from Indianapolis-based Calumet Specialty Products Partners last year for over $490 million. It’s Wisconsin’s only refinery, and it produces gasoline, asphalt and other products.

The refinery had been fined by federal officials several years ago under its previous owners. 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 violations were settled and the problems resolved by the end of that year.

Allen said it was the only OSHA enforcement action taken against Calumet Superior LLC in the past 20 years. Calumet Superior operates as a subsidiary of Calumet Specialty Products Partners, which sold the plant to Husky Energy last year.

The refinery, which dates back to the early 1950s, has a processing capacity of around 50,000 barrels per day and a storage capacity of 3.6 million barrels of crude and products. It processes both heavy crude from the Canadian tar sands in Alberta and lighter North Dakota Bakken crude.