TOKYO — The operator of the crippled Fukushima Daiichi nuclear plant said Wednesday that it had found what it believed was the cause of an extended blackout that disabled vital cooling systems this week: the charred body of a rat.

The operator, Tokyo Electric Power Company, said that when its engineers looked inside a faulty switchboard, they found burn marks and the rodent’s scorched body. The company said it appeared that the rat had somehow short-circuited the switchboard, possibly by gnawing on cables.

The company, known as Tepco, has blamed problems with the switchboard for the power failure that began Monday, cutting off the flow of cooling water to four pools used to store more than 8,800 nuclear fuel rods. It took Tepco almost a day to restore cooling to the first of the affected pools, with cooling of the final pool resuming early Wednesday.

Tepco said it would have taken several days for temperatures in the pools to have risen above the safe level of 65 degrees Celsius, or 149 degrees Fahrenheit. Still, the blackout served as an uncomfortable reminder to many Japanese about the continuing vulnerability of the plant, which had a triple meltdown in March 2011 after a huge earthquake and tsunami knocked out cooling systems. It was the worst nuclear disaster since Chernobyl.