Advance Auto Parts, a leading auto parts retailer, has begun sending letters to customers impacted by a data breach that may have exposed financial information of up to 56,000 people.

The retailer reported March 31 that a "network intrusion" had exposed financial information and was the subject of a criminal investigation. Fourteen of the retailer's stores, including locations in Georgia, Ohio, Louisiana, Tennessee, Mississippi, Indiana, Virginia and New York, are believed to have been affected.

Advance Auto Parts did not specify how customer financial information had been revealed or how access had been gained to its network. In response to the incident, the company notified its credit, debit and check processors.

Customers of the 14 locations listed in an advisory who do not receive a letter can call a toll-free-number provided by the company to find out if they were affected, according to the company.

Company officials said in the advisory that they believed the incident to be contained, and that the company is working with law enforcement and a third-party security expert to uncover what happened.

Advance Auto Parts is offering the affected customers a credit monitoring product from a national credit reporting agency at no cost for one year.

"We sincerely apologize for any inconvenience this attack on our network may cause. Advance Auto Parts has been dedicated for the past 75 years to earning customer trust and for providing Legendary Customer Service," said Darren Jackson, president and chief executive officer, in a statement.