Target stores say up to 40 million card accounts may have been affected by a data breach between Nov. 27 and Dec. 15.

The theft of data took place in Target stores around the country, and may have involved tampering with the machines customers use to swipe cards when making purchases, the Wall Street Journal reported.

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

Officials urged consumers to pay attention to their  credit card statements and check for any suspicious activity. According to reports, every Target store nationwide was affected.