Trading of the stock of Global Payments Inc., which processes credit card transactions, was halted after published reports said it was responsible. The stock fell 9 percent for the day before trading was stopped.
Global Payments did not respond to requests for comment, and Visa would not name the company it believed was to blame. Neither Visa nor MasterCard would say how many customers were affected.
Credit card companies generally protect customers against fraudulent transactions, and Visa said specifically Friday that its U.S. customers were not at risk. Both Visa and MasterCard said their own systems had not been compromised.
Last June, hackers stole information for 360,000 credit card accounts at Citigroup. In the past year, there have been high-profile data attacks against the International Monetary Fund, National Public Radio, Google and Sony's PlayStation Network.