KASHIWAZAKI, Japan (AP): Japanese nuclear regulators discovered Thursday radioactive material had been leaking at least until late Wednesday from an atomic energy plant damaged in an earthquake earlier this week, adding to criticism of the embattled plant operator.

Nuclear inspectors probed the Kashiwazaki-Kariwa plant, which suffered a barrage of leaks and malfunctions in Monday's 6.8-magnitude quake in northwestern Japan. The plant was ordered shut down indefinitely on Wednesday.

The Nuclear and Industrial Safety Agency found radioactive iodine had leaked from an exhaust pipe at the No. 7 reactor between Tuesday and Wednesday night, agency official Hisanori Nei said. Operator Tokyo Electric Power Co. had already announced the release of other radioactive materials from the exhaust vent previously.

The problem may have been caused by a fan inside the building that was not turned off as instructed in an operation manual, TEPCO spokesman Manabu Takeyama said.

The inspectors, however, concluded the leak was too small to harm the environment or public health, Nei said, adding the agency will continue monitoring.