By Stephanie Kelly and Erwin Seba
NEW YORK/HOUSTON (Reuters) - An explosion at Husky Energy's refinery in Superior, Wisconsin, injured at least 10 people, sent smoke billowing into the sky and prompted an evacuation of tens of thousands of residents, local and medical officials said on Thursday.
At least 10 people were taken to area hospitals, one of whom was seriously injured, said a spokeswoman for Essentia Health-St. Mary's Medical Center, which operates hospitals in Superior and nearby Duluth, Minnesota.
What ignited the blast was not clear. After an initial blaze was extinguished, a storage tank was punctured, and a second fire erupted, Husky Energy spokesman Mel Duvall said.
There were no reports of fatalities, and all of the refinery's workers have been accounted for, Duvall said.
Specialists from Williams Fire & Hazard Control of Port Arthur, Texas were hired by Husky Energy and were flying to the scene on Thursday to direct efforts to battle the blaze.
While awaiting their arrival, firefighters were trying to prevent flames from spreading by spraying water on nearby tanks, manager Kollin Schade said at a briefing.
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.
The U.S. Chemical Safety and Hazard Investigation Board has sent a four-person team to investigate. The non-regulatory federal agency investigates serious chemical accidents such as refinery fires.
As a precautionary measure, local officials ordered all residents within a three-mile radius around the refinery to evacuate, Superior Mayor Jim Paine said at a briefing. The city of 27,000 would be evacuated as needed, he said. Friday classes were canceled in Superior and nearby Maple school districts.
"The lights flickered three times and the whole building shook," said Jim Ronning, owner of Hudy's Tavern in Superior, located about a mile (1.6 km) from the facility.
"You can still smell a real caustic smell in the air," said Sara Haugen, owner of Pudge's bar in Superior, located about 135 miles from Minneapolis, Minnesota. "There's a big black plume of smoke coming out.
Husky purchased the refinery from Calumet Specialty Products Partners LP last year, saying at the time that it intended to keep all of its workforce of 180. It produces asphalt, gasoline, diesel and heavy fuel oils, largely using heavy crude oil imported from Canada.
Husky shares fell 8 percent to C$17.54 on the Toronto Stock Exchange, the same day it reported financial results.
(Reporting by Vijaykumar Vedala and Arpan Varghese in Bengaluru, Stephanie Kelly in New York and Liz Hampton and Erwin Seba in Houston; Editing by Chizu Nomiyama, Scott Malone and Cynthia Osterman)