Human error, not equipment failure, may be responsible for the roughly 100 tons of highly radioactive water released from a storage tank earlier this week, the operator of the crippled Fukushima No. 1 power plant says.

When Tokyo Electric Power Co. first disclosed the incident Thursday, it said a faulty valve may have allowed water to flow into the tank, which was already nearly full.

On Friday, however, the utility dismissed that possibility and said it found photographs that showed the valve appeared to be operating properly around the time the leak occurred.