Nine more young people have been diagnosed with thyroid cancer since Japan’s worst nuclear accident two years ago, the Kyodo News agency reported on Wednesday.

That has brought the total number of cancer cases to 12 residents, who were 18 or younger at the time of the meltdowns at the Fukushima Daiichi nuclear plant, it reported, citing unnamed sources.

Fifteen others were also suspected of suffering from some form of cancer, up from seven in a February report, Kyodo said.

But experts at Fukushima Medical University said that it is too early to link the cancer cases to the nuclear disaster. They said the 1986 Chernobyl accident showed that it takes at least four to five years before thyroid cancer is detected.

Local Government officials surveyed about 174,000 young people near the plant in Fukushima.

“Fukushima’s survey examines people who have no symptoms across the board and it is hard to evaluate it because there are no comparable data,” an unnamed Environment Ministry official was quoted as saying. “We need to take a careful look at it.” The Fukushima plant suffered meltdowns at three of its six reactors after it was struck by an earthquake and tsunami. More than 150,000 residents have been forced to leave their homes around the complex due to radioactive contamination.

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