Radioactive cesium has been found on an estimated 200 to 300 tons of wood chips that were left months ago near Lake Biwa in Shiga Prefecture, prefectural officials said.

Samples of the chips show a reading of up to 3,000 becquerels per kilogram, the officials said Tuesday. The readings are below 8,000 becquerels, the threshold requiring special measures such as keeping the contaminated material from coming into contact with water, but exceed 100 becquerels, the maximum allowed for reuse.

The Shiga government started an investigation to determine where the chips came from. They were found in the dry bed of the Kamo River in Takashima and other locations near the lake, officials said.

Some of the chips were laid on a path used for river embankment work and some were found in 77 sandbags, the prefecture said.

Nearby residents alerted the prefecture in late April that the chips had suddenly shown up.

A man suspected of leaving the chips there said he laid them on the path to make it easier to pass through, the officials said. The prefecture has not been able to contact the man since April 30.

The prefecture plans to issue a removal order for the chips, saying that leaving the material there may violate the river law.

“The site is an estuary leading to Lake Biwa, and leaving (the chips) there without permission is extremely malicious. We will deal with the matter strictly,” Gov. Yukiko Kada said.