The massive April explosion at the Husky oil refinery in Superior, Wis., could have been caused by a worn valve that allowed air to leak and mix with combustible hydrocarbons, according to a federal analysis released Thursday.

The explosion caused a raging fire that burned for hours and prompted entire neighborhoods in Superior to evacuate the city for hours. Eleven workers were hurt, and 25 other people also sought medical attention.

The U.S. Chemical Safety and Hazardous Investigation Board on Thursday released a preliminary report on the incident, saying that in a critical portion of the refinery where oil is transformed into gasoline, air could have inadvertently flowed backward through a valve to spark the blast.

The board, which investigates accidents and makes recommendations but issues no fines, termed Thursday's report a "factual investigative update." The board's final report isn't likely to be completed until a year from now at the earliest.

At around 10 a.m. on April 26, an explosion ripped through the refinery owned by Calgary, Alberta-based Husky Energy. Debris from the blast flew about 200 feet and punctured a nearby steel tank filled with asphalt, spilling more than 15,000 barrels, according to the report. Two hours later, the asphalt — another product of the refining process — caught fire.

Neither the explosion nor the fire breached a tank filled with hydrogen fluoride, a highly toxic chemical used at some oil refineries that can vaporize into a gas cloud. A toxic cloud such as that could have killed or injured thousands of people.

The Husky refinery's hydrogen fluoride tank is "in the vicinity" of the asphalt tank that was pierced, Mark Wingard, the safety board's lead investigator, said at a news conference Thursday in Superior.

Still, the agency can't yet conclude that the Superior refinery explosion — with its flying shrapnel — constituted a "near miss" in puncturing the hydrogen fluoride tank, said Kristen Kulinowski, the board's interim executive.

The agency made such a near-miss determination when debris from a 2015 explosion at an ExxonMobil refinery in Southern California nearly pierced a tankful of hydrogen sulfide.

The Superior explosion occurred in the refinery's fluid catalytic cracking unit, where hydrocarbons in petroleum are converted into gasoline and other products. On the day of the explosion, the catalytic cracker was being shut down for routine maintenance. Refinery shutdowns and startups are times of "enhanced risk," as they involve "non-routine procedures," Kulinowski said.

Fortunately, workers on the catalytic cracker were on break when the explosion occurred, and had moved from ground zero — some to blast-resistant buildings, the report said.

At the heart of a fluid catalytic cracking unit — where chemical reactions take place — are two components respectively called a regenerator and a reactor.

"It is important to prevent air in the regenerator from mixing with hydrocarbons in the reactor and downstream equipment because of the potential for such mixing to create flammable [explosive] conditions," the report said.

Valves are used to maintain that separation. However, at times during the Superior refinery shutdown, "conditions existed that could have allowed air from the regenerator to flow backward" through a valve and into the reactor, the report said.

Evaluation of the valve in question "revealed internal wear," the report said.

The unintended mix of oxygen and combustible hydrocarbons appears to have come in contact with deposits of iron sulfide, which are not uncommon in refineries and chemical plants, Wingard said. "The most probable source of ignition are these iron sulfide deposits."

During the maintenance shutdown, Husky had been planning to treat equipment with chemicals to remove any iron sulfide deposits. However, at the time of the blast, the treatment hadn't yet been done, the report said.

The Husky refinery conflagration has similarities to the 2015 ExxonMobil refinery explosion and fire. Due to a valve leak, air and hydrocarbons mixed together and ignited an explosion at the Southern California refinery, the safety board concluded in its final report on that incident.

The Superior refinery isn't expected to reopen for at least 18 months, Husky said in a recent earnings report. The fire and explosion resulted in $27 million in damages and another $53 million in unspecified costs related to the incident.

The refinery was built in the 1950s and was owned for years by Murphy Oil. In 2017, Husky bought the refinery from its then owner, Calumet Specialty Products.