NEW YORK - Tens of thousands of Citigroup customers have been affected by a data breach of Citi cards.

"During routine monitoring, we recently discovered unauthorized access to Citi's Account Online," Citigroup said in a statement. "For the security of these customers, we are not disclosing further details."

Citigroup admitted that the breach was discovered in May, according to the Financial Times.

"We find it peculiar that this information did not come out prior to Wednesday??s vote in the Senate on the Tester-Corker amendment to delay debit card swipe fee reforms," said Gray Taylor, NACS payments consultant and executive director of PCATS, adding, "And we wonder if Citigroup will be fined for not being PCI-compliant."