SUPERIOR, Wis. (AP) — A hole in a valve was the source of an April explosion at a Husky Energy refinery in northwestern Wisconsin that injured 36 people and required the evacuation of a large part of the city of Superior, according to findings of the U.S. Chemical Safety and Hazard Investigation Board presented Wednesday.

According to the update that was shared at a town hall meeting in Superior, erosion created a hole in the slide valve, allowing air to mix with hydrocarbons. The resulting blast sent debris hurtling into an asphalt storage tank. The puncture spilled about 15,000 gallons of hot asphalt, which later ignited and burned for hours.

The report linked the Superior explosion on April 26, 2018, to a 2015 explosion in Torrance, California, the Star Tribune reported . The board says in both cases, an explosive mix of air and hydrocarbons formed inside a fluid catalytic cracking unit because of ineffective safeguards. The unit converts hydrocarbons in petroleum into gasoline.

In both cases, the fluid catalytic cracking units were not in normal operating mode when the explosions happened and were nearing dates for upgrades, the board said.

Several people at the meeting said they want the Superior refinery to stop using hydrogen fluoride to process high-octane gasoline. The highly corrosive chemical can produce toxic vapor clouds.

The tank containing hydrogen fluoride was not damaged by the explosion or fire. While some people who lived nearest to the refinery were told to evacuate due to the presence of hydrogen fluoride, others should have been told as well, said the Rev. Michelle R. Rowell of Concordia Lutheran Church in Superior.

"After that nearest evacuation was done, I think it was irresponsible that the rest of the community was not informed about the presence of that very dangerous chemical," Rowell said.

A spokesman for Husky Energy said Wednesday the Canada-based company will continue working with the board to understand the cause of the explosion.

Husky Energy does not expect to resume normal operations at the Superior refinery until 2020 but says no one will be laid off because of the explosion.

"We've appointed an engineering contractor to oversee design work for the rebuild and as part of that process, we continue to evaluate options around the use of hydrogen fluoride," Husky spokesman Mel Duvall said in an email.