Officials are investigating what they believe is one of the largest-known credit card heists in the U.S. after Mastercard and Visa confirmed the data of as many as ten million customers has been stolen.
 
The companies notified U.S. banks of a potential security breach, and they are now going through the process of pinpointing the affected accounts before contacting customers involved. 

Customers are urged to check their accounts online or contact their financial institution regarding any concerns they have.
 
Visa have already provided the issuing banks with the affected account numbers and assured customers they would not be responsible for fraudulent purchases.
 
Mastercard said in a statement: 'MasterCard is concerned whenever there is any possibility that cardholders could be inconvenienced and we continue to both monitor this event and take steps to safeguard account information.
 
'If cardholders have any concerns about their individual accounts, they should contact their issuing financial institution.'
 
Visa also released a statement saying their customers were victims of data theft, but said its own systems were not hacked.
 
Visa Inc is aware of a potential data compromise incident at a third party entity affecting card account information from all major card brands.
 
Visa has provided payment card issuers with the affected account numbers so they can take steps to protect consumers through independent fraud monitoring and, if needed, reissuing cards.'