OKLAHOMA CITY - Oklahoma City-based fast food chain Sonic has acknowledged a data breach potentially affecting millions of credit and debit cards.

This info comes from security analyst and investigator Brian Krebs.

Multiple financial institutions noticed a string of fraudulent transactions with cards all previously used at Sonic.

In many of these cases, Krebs found the credit card info was being sold underground through the Internet.

When contacted, Krebs reported Sonic stated they were aware of an issue and investigating.

According to the statement, Sonic staff have called in forensic experts and law enforcement to help look into the issue.

There are more than 3,500 Sonic restaurants in 44 states.

Sonic administrators said they don't know how many or which Sonic locations may be affected.

The following is a statement from Sonic:

Thank you for the opportunity to respond to your inquiry. Our credit card processor informed us last week of unusual activity regarding credit cards used at SONIC.

The security of our guests’ information is very important to SONIC.  We are working to understand the nature and scope of this issue, as we know how important this is to our guests.  We immediately engaged third-party forensic experts and law enforcement when we heard from our processor. While law enforcement limits the information we can share, we will communicate additional information as we are able.