Thursday UPDATE(AP) — Target says about 40 million credit and debit card accounts may have been affected by a data breach.

The chain said Thursday that the accounts may have been impacted between Nov. 27 and Dec. 15.

The Minneapolis company said it immediately told authorities and financial institutions once it became aware of the breach and that it is teaming with a third-party forensics firm to investigate the matter.

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.

(AP) — The Secret Service says it is investigating a credit- and debit-card data theft at Target stores.

Secret Service spokesman Brian Leary confirms the agency is investigating, but declined to provide further details.

Minneapolis-based Target Corp. did not respond to requests for comment. A MasterCard representative referred questions to Target.

Target has 1,797 stores in the U.S. and 124 in Canada.