A computer hacker, or hackers, has gained access to the credit card numbers of as many as 5 million credit card customers.

Information was stolen from more than 2.2 million MasterCard International accounts and approximately 3.4 million Visa USA cardholder accounts, according to those companies.

American Express cardholder account information was also accessed, according to Christine Elliott, a corporate spokeswoman for American Express.

Elliott declined to comment on how many American Express accounts were affected.

The theft occurred when the system of a company that processes credit card transactions for merchants was broken into, according to statement released by Visa USA.

Systems operated by Visa and MasterCard were not compromised, and both companies said they have contacted all the financial institutions affected by the theft.

No information was available about which banks were affected by the theft, but a Visa spokesman said that none of the stolen Visa account information has been used fraudulently.

MasterCard is continuing to investigate whether the account information has been used to make purchases, according to Sharon Gamsin, vice president of global communications at MasterCard.

Neither Visa nor MasterCard would identify the company that was hacked, nor would they provide information on how the theft occurred, citing security concerns.

MasterCard became aware of the security breach during the week of Feb. 3, Gamsin said.

Visa and MasterCard, like other credit card companies, use third-party companies to manage credit card transactions between merchants and the financial institutions that issue the credit cards.

Those companies transfer money to a merchant's account and manage any transaction fees, according to a Visa spokesman.

The FBI's Cybercrime division is also investigating the theft, according to Cybercrime division spokesman Bill Murray.

The decision about whether to cancel a credit card account or merely flag it for possible fraud is up to the financial institutions that issued the card, according to Gamsin.

Citizens Bank was notified of the crime by MasterCard on Friday and immediately canceled the affected credit cards to protect its customers, according to Barbara Cottam, director of corporate communications at Citizens Bank, which is part of Citizens Financial Group.

The IDG News Service is a Network World affiliate.

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