Japan’s nuclear safety agency says it is highly likely that radioactive materials are leaking from part of the Number 3 reactor of the troubled Fukushima Daiichi nuclear power plant.

The Nuclear and Industrial Safety Agency spoke to reporters on Friday about an accident in which 3 workers were exposed to radiation in the turbine building of the No. 3 reactor.

It said 3.9 million becquerels of radiation was detected from 1 cubic centimeter of water sampled from the floor of the building. The radiation level was about 10,000 times higher than the water inside a normally operating nuclear reactor.

The agency said the water sample indicated it is highly likely the leak comes from the reactor itself, not from the pool storing spent nuclear fuel.