Whole Foods says it is investigating a data breach that hit an unspecified number of stores, the company said in a statement.

The grocery store chain, which Amazon bought for $13.7 billion in this summer, announced the breach on Thursday--telling customers to closely monitor payment card statements and report any unauthorized charges to the issuing bank. The breach did not affect the primary checkout and payment operating systems, Whole Foods said. The breach only affected in-store taprooms and restaurants payment systems.

According to Whole Foods, the taprooms and restaurants process payments on a separate system from the main checkout lines.

Whole Foods, which has about 450 stores across the U.S., has over 40 locations with a taproom to sell beer, Cnet reports.

It is unclear how many locations were hit and how much data hackers were able to squirrel away. But, the company said "most" of its stores do not have taprooms and restaurants.

"When Whole Foods Market learned of [the breach], the company launched an investigation, obtained the help of a leading cyber security forensics firm, contacted law enforcement, and is taking appropriate measures to address the issue," Whole Foods said in a statement. "The company's investigation is ongoing and it will provide additional updates as it learns more."

Whole Foods made a point to say that the Whole Foods Market payment systems are in no way connected to Amazon.com.

Earlier this week, Sonic, the drive-in burger chain, suffered a credit card data breach, too.