Fast-casual restaurant chain Chipotle Mexican Grill, which has more than 2,000 locations in the United States and other countries, informed customers on Tuesday that its payment processing systems have been breached.

Chipotle said it recently detected unauthorized activity on the network that supports payment processing for its restaurants. The company’s investigation into the incident is ongoing and only limited information has been made public for now.

An initial investigation showed that attackers may have accessed data from cards used at restaurants between March 24 and April 18, 2017, but it’s unclear how many locations are affected.

Chipotle has notified law enforcement and it’s working with cybersecurity firms and its payment processor to investigate the incident. The company believes the breach has been contained, and pointed out that it has implemented some security enhancements.

“Consistent with good practices, consumers should closely monitor their payment card statements,” the company stated. “If anyone sees an unauthorized charge, they should immediately notify the bank that issued the card. Payment card network rules generally state that cardholders are not responsible for such charges.”

Chipotle’s disclosure of the data breach coincided with the company’s financial report for the first quarter of 2017. The restaurant said its revenue increased by 28.1% to $1.07 billion compared to the first quarter of 2016.

Several other major restaurant chains reported suffering data breaches in the past months, including Shoney’s, CiCi’s, Arby’s, Wendy’s and Noodles & Company.

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