WHITING, Ind. (AP) — BP’s sprawling oil refinery in northwest Indiana was hit Thursday by a power outage that prompted the company to shut it down and evacuate workers, authorities said.

Initially, only employees who work at an office building at BP’s Whiting refinery were evacuated. But BP later decided to evacuate all employees out of an abundance of caution, Whiting Mayor Steve Spebar told The Times of Northwest Indiana.

Roads near the refinery were also shut down, and the refinery complex, located along Lake Michigan about 15 miles (24 kilometers) southeast of Chicago, was flaring its stacks in response to the outage, the city of Whiting said in a statement.

“This flaring is a safety release to burn off the extra product and is a normal process during an event,” the city said. “BP is working to resolve the power outage as quickly as possible.”

BP spokesperson Christina Audisho said in a statement that the company was “in the process of safely shutting down the refinery,” and that local fire departments were helping the evacuation by closing roads nearby. “The safety of refinery staff and the community are our highest priority,” Audisho added.

The refinery is the biggest in the U.S. Midwest and sixth largest nationally, processing about 440,000 barrels of crude oil daily, making a variety of liquid fuels and asphalt.