A breach of credit and debit card data at discount retailer Target may have affected as many as 40 million shoppers who went to the store in the three weeks after Thanksgiving, the retailer said Thursday.

Late Wednesday, the Secret Service, which is charged with safeguard the nation's financial infrastructure and payment systems, confirmed it was investigating the breach.

Spokesman Brian Leary declined further comment.

The breach first came to light via a report from respected security researcher Brian Krebs that said Target had suffered a data breach around the time of Black Friday last month "potentially involving millions of customer credit and debit card records."

Target (TGT, Fortune 500), the nation's No. 2 general merchandise retailer after Wal-Mart Stores (WMT, Fortune 500), said cards used at the brick-and-mortar stores between Nov. 27 and Dec. 15, 2013, may have been impacted.

The retailer said it notified authorities and financial institutions immediately after it was made aware of the unauthorized access, and had hired a forensics team to thoroughly investigate how the breach may have happened.

"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," said CEO Gregg Steinhafel in a statement. "We regret any inconvenience this may cause."

The thieves reportedly gained access to data on the magnetic strips of shoppers' cards, potentially allowing them to produce counterfeit versions, according to Krebs.

The thieves could also potentially withdraw cash from ATMs using counterfeit debit cards if they were able to intercept PIN data from Target, he said.