BOISE, Idaho (AP) — An investigation into what sickened several farmworkers in an Idaho hop field earlier this year has closed without a conclusive answer.

State agriculture investigators determined a fungicide applied on a nearby onion field might have been the cause, but they were unable to prove it, the Idaho Statesman reported Wednesday.

The workers on the Obendorf Hop field in Parma fell ill and were hospitalized over Memorial Day weekend.

A crop-duster pilot sprayed a fungicide called Badge SC on the field across the street, investigators said. Several workers told investigators that they felt the spray when the plane passed over them.

The pilot said he turned off the spray when he neared the workers.

Badge SC can cause mild irritation to eyes, skin and lungs, according to the pesticide application label.

Some of the workers had different symptoms. The first worker hospitalized had flu-like symptoms, severe vomiting and stomach aches — symptoms usually linked to a different type of pesticide, investigators said.

Investigators did not find any record of such a pesticide being applied anywhere near the workers at that time. They also were unable to confirm what spray hit the workers because labs were not able to test for Badge SC.

The state's pesticide compliance manager has issued a regulatory letter to the crop-duster pilot. The letter requires the pilot to respond with plans for how to prevent similar incidents from happening.

"Every pesticide applicator has a duty to be careful and to be cognizant and aware of their surroundings and apply it in a manner that is safe, especially when workers are in the area," said Brian Oakey, deputy director at the state agriculture department. "And so that was the regulatory issue that we had with this particular case."

State officials might issue a violation notice or seek other penalties if the pilot's response is not sufficient.

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