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

As a result, 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.

The theft occurred when the system of a company that processes credit card transactions for merchants was broken into, according to a statement released by Visa. Systems operated by Visa and MasterCard were not compromised, and both companies said they have contacted all the financial institutions affected by the theft.

Few Details

No information was available about which banks were affected by the theft, but a Visa spokesperson 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 February 3, Gamsin said.

Under Investigation

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

Because a single vendor handles transactions from a number of different companies, however, it is likely that the thefts were not limited to Visa and MasterCard accounts, according to Gamsin.

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

The decision about whether to cancel a credit card account or merely flag it for possible fraud is up to the financial institution that issued the card, according to Gamsin. Both Visa and MasterCard offer zero-liability policies, which absolve cardholders of responsibility for unauthorized purchases. Cardholders who discover that their account has been used fraudulently should contact the financial institution that issued the card, Gamsin said.

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