On Monday, the parent company of the Sweetbay grocery chain said a security breach of its computer system may have exposed 4.2 million debit and credit card numbers to theft, making it one of the largest such cases in the nation.

Hannaford Bros. says it has secured its credit and debit card transaction system to block future unauthorized access and the Secret Service is investigating. So far, 1,800 cases of fraud are linked to the breach.