A rat caused a 29-hour power cut at Tokyo Electric Power Company's Fukushima Daaichi plant last week.
A photograph was released showing the scorched body of a small rodent found near a temporary switchboard used to supply power to cooling systems for the disabled plant's spent nuclear rods.
Investigators confirmed the power cut began after the rodent touched exposed wires and triggered a circuit breaker.
TEPCO resumed operations by midnight on March 19.
There was no threat of overheating or radiation releases after the outage according to officials.