Chipotle Mexican Grill is investigating a data breach in its payment processing system after reporting a sharp increase in sales.

The fast-casual restaurant said it recently detected unauthorized activity on its network that supports the payment system. Chipotle said the transactions occurred from March 24 to April 18, and the company believes it took the proper precautions to stop the breach.

The breach dominated the company’s other announcement that same-store sales grew 17.8 percent in the first quarter. Compared to the first quarter of 2016, revenue increased more than 28 percent to $1.07 billion. The chain also opened up 57 new restaurants.

Steve Ells, founder, chairman and CEO of Chipotle, said the year "is off to a strong start, as our restaurant managers and teams are energized by our renewed focus on the customer."

He added: "By simplifying the focus in our restaurants to only those elements that lead to a great guest experience, our operations have improved every single month, which gives us confidence that we are on our way to achieve our mission to ensure that great food made with whole unprocessed ingredients is accessible to everyone.”

The increase in sales comes after Chipotle’s flurry of food contamination incidents a couple years ago.