Nationwide retail giant Target is investigating a data breach potentially involving millions of customer credit and debit card records, multiple reliable sources tell KrebsOnSecurity. The sources said the breach appears to have begun on or around Black Friday 2013 — by far the busiest shopping day the year. According to sources at two different top 10 credit card issuers, the breach extends to nearly all Target locations nationwide, and involves the theft of data stored on the magnetic stripe of cards used at the stores. Minneapolis, Minn. based Target Brands Inc. has not responded to multiple requests for comment. Representatives from MasterCard and Visa also could not be immediately reached for comment.