The operator of the Fukushima No. 1 power plant said that visual checks of the crippled complex’s leaking cisterns have failed to turn up any clues as to how radioactive water is escaping.

Although workers removed some of the soil covering one of the troubled sunken reservoirs and checked an area where a leak detection pipe passes through one of the three water-containment sheets, no irregularities were found, Tokyo Electric Power Co. reported Friday.

“We have no choice but to think that the leakage occurred at another place in the case of the No. 2 (cistern),” Tepco spokesman Masayuki Ono said at a news conference.

Some of the junctions between the water-containment sheets might also be suspect, but even if holes exist in these areas they are unlikely to be large based on the amount of water that has accumulated in the detection pipe, according to Ono.

The tainted water in the pipe has been returned to the No. 2 cistern, he said.