The U.S. Secret Service is investigating a security breach at Global Payment, the country's third-largest processor of electronic transfers, that may have put tens of thousands of credit cards at risk for fraud.

MasterCard and Visa alerted the nation's banks, but said their own systems were not compromised.

"MasterCard is concerned whenever there is any possibility that cardholders could be inconvenienced and we continue to both monitor this event and take steps to safeguard account information.  If cardholders have any  concerns about their individual accounts, they should contact their issuing financial institution," MasterCard said in a statement.

MasterCard and Visa have told financial institutions that issue credit cards which account numbers have been affected, but there's no official word how many cards may be at risk.

"Visa Inc. is aware of a potential data compromise incident at a third party entity affecting card account information from all major card brands. There has been no breach of Visa systems, including its core processing network VisaNet," the company said in a statement.

Trading of Global Payment stock was halted at 11:50 a.m. after shares dropped 9 percent.

The company issued a statement noting that it identified in early March that "card data may have been accessed."

"It immediately engaged external experts in information technology forensics and contacted federal law enforcement. The company promptly notified appropriate industry parties to allow them to minimize potential cardholder impact," the company said in a statement.

Global Payment's Chairman and CEO Paul R. Garcia said, "It is crucial to understand that this incident does not involve our merchants or their relationships with their customers."

The breach was first publicly reported by Brian Krebs, who writes the blog Krebs on Security.  He said the compromised cards all had something in common:  they were used at parking garages in New York City.

The Secret Service declined to comment about specifics, saying investigators are in the early stages of their work.

"Although these cases can be technically complex, the Secret Service relies on a mixture of traditional investigative methods like interviews, surveillance, evidence analysis, and search warrants, along with sophisticated computer forensics to resolve these cases," spokesman Brian Leary said.