Visa, Mastercard and Discover have warned that credit card holders' personal information could be at risk after a security breach.
The firms said there had been "no breach" of its own system, instead blaming a third party.
Security blog KrebsOnSecurity, which first reported the story, said industry sources believed more than 10 million cards may have been compromised.
Reports suggested the stolen details had been obtained in New York.
The Wall Street Journal quoted its own industry sources as saying card-processing firm Global Payments was the company that suffered the breach. Shares in the company fell by more than 9% on Friday.
Global Payments has not responded to requests for comment.
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