Credit card issuer DC Card Co. said Tuesday that personal information on as many as 477,959 clients might have been leaked.

The individuals are among those who became DC Card holders before Sept. 20, 2001, and were born between Sept. 20, 1947, and Sept. 20, 1967, the company said. The information includes names, addresses and phone numbers, and may include card numbers and their amount of use.

DC Card began in-house investigations last month into the possibility that personal information on 34 card holders had been leaked.

The 34 people were found to be among 477,959 individuals registered on a database that DC Card created in October 2001 in connection with direct mail to promote insurance products. It is possible information on all of the individuals in that database was leaked, the company said.

The information was copied from the company’s host computer and stored in a terminal at its office but was never provided to people outside of the firm, according to DC Card. It said it is looking into the possibility its employees were involved in the leak.

DC Card, an affiliate of the Bank of Tokyo-Mitsubishi, said it is working with police to confirm the routes of the possible leakage.

“We will severely punish those responsible, including top management” if the investigation confirms a leak, DC Card President Akira Katayanagi said.

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