Fukushima in northern Japan says cattle shipped from a farm in the prefecture were highly likely to have been fed hay contaminated with radioactive cesium.
The prefecture reported the results of its investigation to the national government on Monday in connection with new reports of contaminated beef found at a food processor.
It launched its investigation on Friday, just after beef from 12 head of cattle shipped from a farmer in April was found to contain radioactive cesium at twice the government safety standard.
In response, the national government immediately put off lifting a ban on shipments of beef and cattle from the prefecture.
Fukushima says the farmer told it that he had kept imported hay in his cattle barn with no side walls before he evacuated in the wake of the accident at the Fukushima Daiichi nuclear power plant.
It says the farmer fed his cattle the hay a week later when a very high level of atmospheric radiation was still detected. The prefecture concluded that the hay had been contaminated with atmospheric radioactive cesium.
Fukushima plans to ask the government to lift the shipment ban as soon as possible, saying the government did so for cattle fed contaminated rice straw from other prefectures last week.