Willmar, Minn. (WCCO) – Investigators in Willmar say they may never know what caused a mass illness inside a Jennie-O turkey plant.

The sudden outbreak of breathing difficulties and nausea hit 28 female employees inside the plant, many of whom were taken to a local hospital.

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Paramedics and hazardous materials crews were called to the turkey processing plant around 7:15 p.m. Friday.

The first sign of trouble hit shortly after dinner time. That’s when two female workers inside plant No. 4 became suddenly ill and were taken outside.

“The complaint was someone was having difficulty breathing and coughing,” explained paramedic Brad Hanson.

Hanson is the emergency medicine manager for Willmar Ambulance Service and was among those first arriving on the scene. Hanson said as soon as he arrived, more and more women began coming out of the plant.

They were vomiting and coughing, experiencing difficulty breathing. None of the patients was experiencing fever.

Hanson and the other paramedics on the scene set up emergency triage to assess the most ill. Firefighters and more ambulances were called in, along with a school bus to help organize the scene.

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“We were trying to deal with a very large crowd,” Hanson said. “There were people all sick and throwing up, and we were trying to put triage tags on them.”

Seven of the female workers were treated and released at the scene. But 21 others, who were more seriously ill, were transported by ambulance to Rice Memorial Hospital in Willmar.

The high number of patients led to a chaotic scene, as a large number of family members and friends of the sick poured in after learning what had happened.

The patients were given fluids, oxygen and anti-nausea medications to ease their discomfort. Meanwhile, Willmar Fire Department’s hazardous materials unit and Jennie-O investigators searched for a cause, but came up empty.

“They didn’t find any equipment failures and the fire department didn’t find any gas, no chlorine, no ammonia. Nothing of the normal things you would have at a processing plant that would make anybody sick,” Hanson adds.

The plant has experienced recent cases of employees with the flu. However, the suddenness of the onset and the large numbers affected at the same time don’t fit a typical flu outbreak.

The plant has had large illnesses twice in the past 10 years. They were later linked to carbon monoxide and ammonia leaks.

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State OSHA investigators and Jennie-O plant safety officials will continue the investigation into what caused the sickness.