Citibank's nationwide system of 2,000 automated teller machines crashed on Tuesday.
The cause of the outage, which began at 5 p.m. and lasted about four hours, was due to an internal software problem, Citibank spokesman Mark Rodgers said.
About 2 million U.S. households hold consumer banking accounts at Citibank, he said.
"Citibank sincerely regrets any inconvenience this temporary interruption may have caused its customers," Rodgers said.
Citibank said it will waive any fees its customers incurred because of the outage, including convenience fees charged by competing banks whose ATMs are used by Citibank customers.
Citibank, a unit of financial services giant Citigroup, is one of the largest consumer banks in the world.
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