The U.S. Secret Service is investigating a reported credit card data hack at nationwide retailer Target.

Target stores were hit by an extensive theft of its customers' credit card and debit card data, beginning on or around Black Friday, the Wall Street Journal and several security blogs reported Wednesday.

The thefts took place in stores nationwide. The thefts may have involved tampering the software on the machines customers use to swipe cards when making purchases, the newspaper reported.

The credit card security breach means that millions of cardholders could be at risk.

The thieves gained access to data that is stored on the magnetic stripe on the back of the credit and debit cards, according to the newspaper.

Customers who shopped the website online were not affected.

Secret Service spokesman Brian Leary said Wednesday the agency is looking into the situation.

Target Corp. said that customers who made purchases at its U.S. stores during the impacted period and suspected unauthorized activity should call them at (866)852-8680.