Four giant card-payment processors and large U.S. banks that issue debit and credit cards were hit by a data-security breach after third-party services provider Global Payments Inc discovered its systems were compromised by unauthorized access.

It was not immediately clear how many cardholders became victims of the breach, which affected MasterCard Inc, Visa Inc, American Express Co and Discover Financial Services, as well as banks and other franchises that issue cards bearing their logos.

U.S. law enforcement authorities, including the Secret Service, are investigating and MasterCard said it has hired an independent data-security organization to review the incident.

Global Payments said it determined that an unauthorized entity had accessed its systems and possible customer card data in early March. Krebs on Security, a blog that first reported the incident on Friday, said accounts had been compromised for more than a month, between January 21, 2012 and February 25, 2012.

Global Payments is holding an investor conference call Monday morning to discuss the issue.