A magnetic tape kept in Singapore that contains information on 123,690 customer accounts at Citibank’s Japan unit has been missing since last month, officials at the Japan arm of the U.S. bank said Friday.

They said it was not immediately known whether the data have been leaked to a third party. Citibank will send a written apology and explanation the each customer whose information is missing, the officials said.

The bank’s Japan unit said, “We will review our management process so that such an incident will never occur again.”

The bank will assume responsibility and take appropriate steps if customers suffer any disadvantage from the incident, Citibank said.

Financial Services Minister Heizo Takenaka said the Financial Services Agency is questioning the bank and has ordered it to file a report on how the incident happened and how it will handle it.

“It is very regrettable that such an incident has occurred at a financial institution backed by public trust,” Takenaka said.

He told reporters the FSA will move to prevent similar incidents at other financial institutions operating in Japan.

Citibank’s Japan unit said the tape disappeared on Feb. 21 while it was being transported by a Singapore security company to a data-processing center that manages customer information for Japan and other Asian economies.

The tape had backup information of monthly bank statements, including customers’ names, addresses, account numbers and account balances. Their personal identification numbers were not included.

The Japan unit said the data of the missing tape were strictly protected, and a third party can only read the information using a specific computer system.

The bank claimed the tape does not contain information that a third party could use to withdraw cash from customer accounts.

Citibank reported the loss of the tape to Singapore police. It reported it to the FSA in late February.

The bank’s Japan arm, based in Tokyo’s Minato Ward, has 20 branches and five sub-branches nationwide. It does not disclose the number of its clients.

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