This week's power outage at Japan's tsunami-crippled nuclear plant may have been caused by a rat.

Masayuki Ono, spokesman for Tokyo Electric Power Co., the utility that runs the Fukushima Dai-ichi plant, says a 15-centimeter (6-inch) rat was found dead Wednesday near a switchboard. He says the rat may be linked to the power failure, but that more investigation is needed to be sure.

Cooling systems at the plant for four storage pools for nuclear fuel were knocked out Monday. Power was restored early Wednesday at all nine affected facilities.

The power outage was a reminder that the world's worst nuclear disaster since Chernobyl is far from resolved. Tsunami-damaged backup generators set off the March 2011 disaster. Decommissioning the Fukushima reactors is expected to take decades.