A spokesman for MasterCard confirmed that the company is investigating a potential security breach involving a Michigan retailer. MasterCard has notified the affected banks to watch for any suspicious account activity "and to take the necessary steps to protect cardholders," the spokesman said in an e-mailed statement.

"MasterCard is concerned whenever cardholders are inconvenienced, and we will continue to monitor this event," the statement said.

Fifth Third is among several banks and credit unions in the Muskegon area that have been forced to block and reissue credit and debit cards because of fraud concerns that appear to be related to a breach at Wesco that occurred between July 25 and Sept. 7. A statement posted on Wesco's Web site said the company is investigating the possibility of credit card fraud associated with card use at its stores. Both the U.S. Secret Service and the U.S. attorney's office are investigating the breach, Wesco said.

Wesco itself has not offered any explanations as to how the breach may have occurred. But very often such data compromises involve security breaches at point-of-sale (POS) systems, said Avivah Litan, an analyst at Stamford, Conn.-based Gartner Inc.

"Four out of five data breaches are happening at the point-of-sale system," Litan said. Especially vulnerable to such breaches are systems at convenience and grocery stores, as well as gas stations, she said.