Investigators have not been able to figure out what caused 24 workers at the Jennie-O turkey processing plant in Willmar to suddenly fall ill last October.

According to the West Central Tribune, the Occupational Safety and Health Administration (OSHA) did not find any health hazards or violations at the plant, nor any abnormal chemical levels within the building.

"The letter from OSHA confirmed our original finding that there was nothing out of the ordinary in the environment of the Willmar Avenue Plant on the evening of Oct. 17," Pat Solheid, Jennie-O's vice president of human resources and administration, told the newspaper.

Multiple workers at the Jennie-O No. 4 plant fell ill, which lead to 24 people requiring treatment in an emergency room, with one of them admitted.

The plant was evacuated and temporarily shut down following the outbreak, after employees were found to be coughing and vomiting.

An investigation by local officials in the aftermath of the breakout also found no cause for the sudden illness, with tests for carbon monoxide, carbon dioxide and ammonia all coming back negative, a finding that has been confirmed by OSHA.