(Reuters) Payment card data was stolen from an unknown number of Target Corp customers starting on the busy Black Friday weekend in a major breach at the U.S. retailer, according to a person familiar with the matter.

The Secret Service is investigating, according to a spokesman for the agency, which safeguards the nation’s payment systems. Target officials did not respond to requests for comment.

Investigators believe the data was obtained via software installed on machines that customers use to swipe magnetic strips on their cards when paying for merchandise at Target stores, according to the person who was not authorized to discuss the matter and declined to provide further details.

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