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Residents are still not in their homes after a smoky refinery fire forced most of their northwestern Wisconsin community to evacuate.

Investigators on Friday searched for the cause of a blast at a Husky Energy refinery that injured at least 10 people and rocked a northern Wisconsin city, sending smoke billowing into the sky and prompting tens of thousands to evacuate.

Multiple people were injured following an explosion and fire at an oil refinery in Wisconsin on Thursday morning, officials said. No deaths were reported, and officials said all workers had been accounted for.

An explosion and asphalt fire at a Wisconsin oil refinery on Thursday sent huge plumes of smoke into the air that pollution experts said nearly certainly contains large amounts of toxins, posing a serious health risk to those living downwind.

Douglas County Board chairman Mark Liebaert said he believed the U.S. Environmental Protection Agency was one of those agencies. The non-regulatory federal agency investigates serious chemical accidents such as refinery fires.

In Duluth, spokeswoman Jessica Stauber says St. Luke's Hospital is treating one person.

Kara Tudor, 30, and Julia Johnson, 27, live about 2 miles from the refinery and were ordered to evacuate. They quickly scooped up their two dogs and three cats, grabbed their toothbrushes and drove to a friend's house in Duluth, where they were watching the news for updates.

Eric Mathews, a boilermaker for Wales, Wis. -based CTS Inc. that has contractors working inside the refinery, said he was about 200 yards away on break when the initial blast occurred, according to the Duluth Tribune.

"It felt like a truck hit the building", she said. "It was a stressful moment", Johnson said. She also wondered if a plane had crashed and feared her house might collapse. Johnson looked around but didn't see anything - then a few moments later saw dense black smoke rising over the trees.

It isn't clear how many people the evacuation order will effect. At the time of the explosion, crews were in the process of shutting down the refinery so that repairs could be made to the decades-old facility. "We threw our dogs in [Johnson's] auto". I put the cats in my vehicle, ... we grabbed our toothbrushes and just left.

We'll have more information as it becomes available.

Tudor, a scientist at a research company, said residents haven't been told much about the potential danger, what is in the smoke or why it is so black. It's created to protect vessels and people from possible air quality hazards, the military said. "If you're in the smoke plume, there is at least some health risk", said Paine.

Owned by Alberta-based Husky Energy, Wisconsin's only refinery produces gasoline, asphalt and other products.

Husky purchased the refinery last year from Calumet last year.

Calgary-based Husky Energy bought the refinery from Indianapolis-based Calumet Specialty Products Partners previous year for more than $490 million.