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The Associated Press
This Thursday, June 1, 2017, photo provided by Jeff Lange shows the scene following a fatal explosion and fire at the Didion Milling plant in Cambria, Wis. Recovery crews searched a mountain of debris on Thursday following a fatal explosion late Wednesday at the corn mill plant, which injured about a dozen people and leveled parts of the sprawling facility in southern Wisconsin, authorities said. (Jeff Lange via AP)

MADISON, Wis. — Crews searched the ruins of a southern Wisconsin corn mill on Friday for a worker who remained unaccounted for following a fatal explosion at the facility.

The blast occurred late Wednesday at the Didion Milling Plant in Cambria, a rural village about 45 miles (72 kilometers) northeast of Madison. At least two workers were killed and nearly a dozen were taken to hospitals following the explosion and fire, which also leveled parts of the plant.

Local fire department officials said the search for the last missing worker was ongoing Friday at the site, according to Cambria Village Clerk Lois Frank. None of the victims has been identified.

The cause of explosion remained unclear Friday. A smaller fire occurred in a different part of the facility on Monday, and investigators were working to determine whether it may be linked to Wednesday's explosion, said Cambria Fire Chief Cody Doucette.

The plant was reprimanded by federal safety inspectors in 2011 for not taking precautions against dust explosions, which are a major hazard in handling grain, according to U.S. Occupational Safety and Health Administration records. OSHA investigators are investigating the latest fire.

"The safety and security of our employees is our top priority," Didion Vice President of Operations Derrick Clark said in a news release. "Over the past 44 years, the Didion team has grown to be a close-knit family, and we ask for your prayers during this difficult time."

The plant, which processes corn for ethanol and other uses, was cited in January 2011 for exposing its workers to dust explosion hazards. The plant's filters lacked an explosion protective system, according to OSHA records.

The federal safety agency ordered the mill to correct the problem by April 2011, and the records show Didion paid a $3,465 fine and the case was closed in September 2013. OSHA hasn't cited the plant for anything since, the records show. Clark's statement didn't address the past citation.

Dust explosions can occur when high concentrations of dust particles are suspended in the air in a confined space during grain handling. A spark from something like a cigarette butt ignites it, according to the U.S. Department of Agriculture.

There were five grain dust explosions in the U.S. last year, including two that were fatal, according to a Purdue University annual report. Keeping facilities clean of dust and equipment in good working order to reduce the possibility of igniting the dust are critical to preventing explosions, the report said.

Emails sent to several Didion officials Thursday and a voicemail left for Vice President of Sales Jeff Dillon by the Associated Press weren't returned. A note posted on the company's website said the company would be closed until further notice.

Columbia County Sheriff Dennis Richards said 16 people were in the plant at the time of the explosion. Authorities said early Thursday that one person had been killed, and Richards reported later in the day that another worker had been found dead in the rubble. Eleven people were taken to area hospitals via ambulance and helicopter.

Nearly two dozen fire departments and four police departments responded to the Wednesday explosion. Cambria Village President Glen Williams said the fire was contained by early Thursday and there were no evacuations in the area.

Cambria Fire Chief Cody Doucette said some area residents briefly lost power after the blast. Schools in the Cambria-Friesland district were closed Thursday as a precaution, but classes were set to resume Friday with counselors on hand for students, Superintendent Timothy Raymond said.

The corn mill is an economic anchor for Cambria, a community of about 770 people, according to Williams.

"Quite a few of the employees live in the village and surrounding area. So it's going to affect the whole area. Not just the shock of the event, but the economic hardship to the families," the village president said.

The company employs more than 200 people. It has offices and a soybean plant in Jefferson County to the southwest, the mill and an ethanol plant in Cambria and an oil packaging plant in Green Lake County to the north, according to the company website.

Brothers John and Dow Didion began Didion Milling in 1972 and construction on the Cambria corn mill was completed in 1991, according to the website. The company's corn products are used in brewing beer as well as in making chips, breakfast cereals, bathroom moldings, steel and ethanol.

Associated Press writer Gretchen Ehlke in Milwaukee contributed to this report.