TJX Cos. (TJX), operator of T.J. Maxx and Marshalls discount stores, said Wednesday its computer systems were hacked late last year and customer data has been stolen.

The company said the full extent of the intrusion is not yet known, but it is conducting a full investigation.

The hackers broke into a system that handles credit and debit card transactions, as well as checks and merchandise returns for customers in the U.S., Puerto Rico, and may also involve customers of T.J. Maxx stores in the U.K. and Ireland.

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The break-in was discovered in mid-December, but was kept confidential upon the request of law enforcement officials.

TJX said it has hired General Dynamics Corp. (GD) and IBM Corp. (IBM) to upgrade its security system.