The highly radioactive water leaking from the No. 3 building of the ravaged Fukushima nuclear plant is probably the water that is used to cool melted nuclear fuel, operator Tokyo Electric Power Co. announced Sunday.

“The leaked water is highly likely to have come from the water that was already used to cool fuel rods, and not from leaked rainwater or cooling water (on its way to the reactor),” stated a TEPCO official, according to The Asahi Shimbun.

TEPCO reportedly made the determination when samples of the leaked water were found to be hotter and more radioactive than rainwater and unused cooling water.

The new revelations follow TEPCO's announcement on Saturday that a 30 centimeter leak was found by workers viewing a video feed that showed the inside the No. 3 building. The company claimed that the water was leaking into the basement of the building but not beyond its structure.

At the time, TEPCO officials said they did not know the source of the leak. Media reports suggest that leaking cooling water is one of the more dismal potential causes.

"If the leak is from water being used to cool the reactor, it would be highly contaminated and a new headache for TEPCO and the government," reported The Asahi Shimbun on Sunday, discussing the potential cause of the leak before it was known. "A series of leaks from storage tanks of water that had been used to cool the damaged reactors and problems with groundwater entering reactor buildings and mixing with radioactive water there has hampered the plant's decommissioning process."

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