Target acknowledged Thursday that data connected to about 40 million credit and debit accounts was stolen between Nov. 27 and Dec. 15. Officials say the problem was identified and eliminated.
The theft was the second largest credit card breach in U.S. history, exceeded only by a 2005 scam involving retailer TJX Cos, which runs stores like T.J. Maxx and Marshall's. That incident affected 45.7 million accounts.
On Friday, Target said they have received "very few reports of actual fraud, but they continue to monitor the situation. The retailer also stressed that consumers will not be financially responsible for fraudulent charges.
The Secret Service and a private forensic team hired by Target are investigating whether the theft was an inside job, or if it was organized from outside -- possibly even overseas.
Avivah Litan, a security analyst with Gartner Research, said she believes the theft may have been an inside job because companies like Target spend millions of dollars every year on its credit-card security measures.
"The fact this breach can happen with all of their security in place is really alarming," Litan said.
Massive security breaches like this one are generally "heavily organized and sophisticated," according to Jason Oxman, CEO of the Electronics Transaction Association.
Target says the company is investigating, but it has not disclosed exactly how the breach occurred. Customers who see suspicious charges on their statements were advised to report them to their credit card companies and call Target at (866) 852-8680.
"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," Chairman, President and CEO Gregg Steinhafel said Thursday in a statement.
But on Target's Facebook page, angry Target customers complained that they had trouble contacting someone on its website and call centers.
"Why don't you just cancel everyone's card and then spend your man power reissuing them to everyone since none of us can get through using the number of website," one person posted on the Target Facebook page.
Target apologized and said it is adding more workers to field calls and help solve website issues. The company announced Friday that it will be offering customers a 10 percent discount this weekend, and it is also offering free credit monitoring services.
ABC News and The Associated Press contributed to this report.