Questions about the use of a highly toxic chemical at an oil refinery in Superior, Wis., persisted Monday as Gov. Scott Walker and other officials toured the site of last week's massive explosion and fire.

While investigators continued to look for the cause of Thursday's blast that sent thousands fleeing under a mandatory evacuation order, a spokesman for refinery operator Husky Energy Inc. didn't answer questions about whether the company plans to use the toxic hydrogen fluoride in the future.

The company remains focused on the investigation and cleanup for now, and it will later evaluate its use of hydrogen fluoride, said Husky spokesman Kollin Schade.

Numerous refineries use the chemical to increase gasoline's octane, but it's a poisonous gas that has raised alarms for years. The chemical forms dense vapor clouds that can travel for miles near ground level.

A 2011 report from the Center for Public Integrity, a nonprofit news organization, said that in a worst-case scenario, the amount used at the Superior refinery could seriously injure or kill up to 180,000 people in the Twin Ports area if the tank holding the hydrogen fluoride were compromised.

Calls for petroleum companies to use an alternative have come from numerous groups over the years, including the United Steelworkers, the union representing many refinery employees.

Some 50 refineries in the United States use it despite safer alternatives, making some 26 million people nationwide vulnerable to leaks, said a 2013 report from the Steelworkers. "No other chemical operation puts as many people at risk," the report said.

In a three-year span, the union recorded some 131 incidents or near misses involving hydrogen fluoride that injured 24 workers and sent six to the hospital.

Superior Mayor Jim Paine said Monday that he plans to meet with Husky CEO Rob Peabody this week.

"I'm going to ask a lot of questions about HF," he said, adding that he wants to know how common it is in the industry, how expensive it would be to use something else, and what logistical challenges the company would face. "Based on that information, I will make a decision about whether or not I need to be publicly supporting a change."

Paine said the city incurred extra, but manageable, costs due to the fire, mainly in firefighters' overtime. He said he doesn't yet know the exact amount, but Husky Energy has offered to reimburse the city.

Some residents who feel they suffered financial losses due to the fire have already been in contact with Husky Energy, the mayor added.

Investigators now have access to the refinery, but it's not known how long their work will take or when the refinery might reopen.

Walker toured the facility Monday. Paine said he and others were allowed to go near the fire scene, where he saw blackened tanks, ruined insulation and flashing and other evidence of the fire.

Thirteen people were injured when the refinery was rocked by an explosion Thursday morning that was felt for miles. A fire broke out two hours later when asphalt pouring from a storage tank caught fire. The blaze threw huge plumes of black, oily smoke into the air, prompting the city to order a mandatory evacuation for some neighborhoods.

The fire burned for hours before firefighters extinguished the blaze that evening.

The evacuation order, for everyone within 3 miles of the refinery and 10 miles to the south, was lifted early Friday.