Target says it immediately notified authorities and financial institutions when it became aware of the breach. The chain has a third-party firm investigating the incident. The Secret Service confirmed to ABC News on Wednesday they are investigating the massive breach.
The stolen data includes customer names, credit and debit card numbers, card expiration dates and the three-digit security codes located on the backs of cards.
Experts say it is unlikely the credit card reader was tampered with due to the size of the breach.
Target says the data breach did not affect online purchases. Since the breach was not online, but in the stores themselves, thieves gained access to critical information.
"If the bad guys can steal that information off the back of the card, they can actually create a second copy of it," said security expert Brian Krebs.
Target CEO Gregg Steinhafel issued the following statement regarding the massive data breach: "Target's first priority is preserving the trust of our guests and we have moved swiftly to address this issue, so guests can shop with confidence. We regret any inconvenience this may cause."
Target is advising customers to check their credit card statements carefully. Any customer who notices suspicious charges or suspect any unauthorized activity on their cards was urged to call the company at 866-852-8680.
The discount chain has 1,797 stores in the United States.
In the meantime, Ken Chaplin of the Experian Credit Reporting Agency and protectmyID.com, advised consumers to use credit cards instead of debit cards because it has more protections. He also urged consumers to keep their receipts for ATM and credit card purchases.
For more advice from the FTC on protecting your identity, visit www.consumer.ftc.gov/features/feature-0014-identity-theft
ABC News and The Associated Press contributed to this report.