NEW YORK (CNN) -- The hacker who breached a security system to get into credit card information had access to about 5.6 million Visa and Mastercard accounts, far more than originally announced, the two card associations told CNN Tuesday.

Monday, Visa and Mastercard said the hacker could look at as many as 2.2 million accounts after breaching the security system of a company that processes credit card transactions on behalf of merchants.

None of the original set of compromised Visa cards had been used fraudulently, Visa spokesman John Abrams said Monday. A Mastercard spokeswoman could not say whether any of their cards had been used fraudulently.

The affected accounts make up almost 1 percent of the 574 million Visa and Mastercard cards in the United States. Spokesmen for the two associations said Monday they promptly notified the banks that issued the affected cards.

Both card companies have zero-liability policies, which protect cardholders from responsibility for any unauthorized or fraudulent charges.

Citizens Bank, a financial institution serving the Northeast, shut down the accounts of 8,800 customers whose card numbers had been accessed after being notified by Mastercard last Friday, bank spokeswoman Pamela Crawley said Monday. All of those accounts were safe, she said.

The FBI is aiding in the investigation.