Husky Energy is investigating a noise that prompted the evacuation of its refinery in Superior, Wis., on Thursday.
After a loud pop came from a tower, employees were evacuated and power and gas were shut off to the refinery "as a precautionary measure."
"To be clear, even in a scenario where the tower collapses there is nothing around the tower which could be struck which would cause a danger to the community," the Superior Police Department said in a news release, adding there is "no risk of fire, explosion, release of any chemical or other danger to the public."
A road near the refinery, Stinson Avenue, was closed, and police have not said when it will reopen.
Husky said the stack in question "normally has some movement in the wind, however out of an abundance of caution we removed people from site while we confirm the integrity of the structure."
"Working with the Superior police, we have used drones to take a preliminary look at the stack, and there is no evidence of damage," Husky spokeswoman Kim Guttormson said Friday. "We are developing a plan to determine our next steps and further investigate."
A worn valve caused an explosion and fire at the refinery in 2018. Dozens of people were injured, and most of Superior's 27,000 residents were asked to evacuate due to the threat of a hydrogen fluoride release, which did not come to pass.
The refinery is currently being rebuilt and is expected to resume normal operations in 2021.