Hackers have raided a US supermarket chain stealing 2.4 million credit and debit card numbers
Schnucks Markets for up to four months to 29 March possibly by data-stealing malware which was detected on the network.
Mandiant was recruited after fraud was detected on customer cards.
Fraudsters masquerading as breach investigators had contacted victims requesting personal information.
"Over the years, technology has helped us deliver superior customer service, but it also introduces risks that we have actively worked to manage through compliance audits, encryption technology and various other security measures," CEO Scott Schnuck said.
The company was also is facing a lawsuit, according to news reports.