(Number of people injured raised to 20)
By Brendan O’Brien and Erwin Seba
MILWAUKEE, Wis./HOUSTON, April 27 (Reuters) - Tens of thousands of residents of a northern Wisconsin city were cleared to return to their homes on Friday, a local official said, a day after a Husky Energy Inc refinery explosion injured at least 20 people.
The facility, which refines 38,000 barrels of oil a day, was shut on Friday as firefighters monitored hot spots from the fire within the plant, said Husky spokeswoman Kim Guttormson.
Only one person injured in the blast remained in Essentia-St Mary’s Medical Center in Duluth, Minnesota, a hospital spokeswoman said on Friday. The person was listed in good condition.
“All indications are that the refinery site is safe and stable and the air quality is clean and normal,” Superior Mayor Jim Paine said on Facebook. The city of about 27,000 people lifted the evacuation order as of 6 a.m. local time (1100 GMT).
It was too early to determine the cause of the blasts or the extent of damage to refinery production units at the site, which employs about 165 people, Guttormson said.
Fifteen other people who were treated for blast-related injuries have been released from Essentia Health hospitals, the hospital spokeswoman said.
Melissa Burlaga, a spokeswoman for St. Luke’s, a healthcare system in nearby Duluth, Minnesota, said four people were treated and released on Thursday.
The cause of the explosion was not clear. After an initial blaze was extinguished, a storage tank was punctured, and a second fire erupted.
Thick black smoke billowed from the facility and hung over Superior throughout the day on Thursday, forcing tens of thousands to flee homes and businesses.
Friday classes were canceled in Superior and nearby Maple school districts.
The refinery had additional workers on site preparing for a plant-wide overhaul when the blast occurred, Husky said. It produces asphalt, gasoline, diesel and heavy fuel oils, largely using heavy crude oil imported from Canada.
The U.S. Chemical Safety and Hazard Investigation Board sent a four-person team to investigate the blasts. The non-regulatory federal agency investigates serious chemical accidents such as refinery fires.
Husky purchased the refinery from Calumet Specialty Products Partners LP last year. (Additional reporting by Stephanie Kelly in New York and Bernie Woodall in Fort Lauderdale, Fla. Editing by Kenneth Maxwell, Scott Malone and Richard Chang)