NTT DoCoMo Inc. said Monday the phone numbers and other personal data of about 24,600 of its clients have been leaked.

The mobile phone operator said it has launched in-house investigations because there is a high possibility the data were leaked by insiders.

The information appears to be partial lists of customers in Niigata Prefecture and neighboring areas, with names, addresses, mobile phone numbers and phone numbers used for inquiries.

According to NTT DoCoMo, the information appears to include lists of customers whose mobile phone fees were reduced or exempted after powerful quakes struck Niigata Prefecture in October.

On Thursday, NTT DoCoMo announced that a list of about 300 customers had been leaked, but that number swelled after the company investigated further.

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