The issue at Neiman appears to have gone on for significantly longer than the widespread attack on Target. In Target's case, however, the data that was stolen appears to be much more significant and ripe for fraud. Target has said card numbers from 40 million customers were stolen, along with encrypted PINs for debit cards. It also estimated that other personal information belonging to 70 million people had been stolen by the hackers.
Neiman Marcus said on Thursday that it had "no knowledge of any connection" between its data breach and Target's.
Neiman has not publicly given any estimate of how many credit card numbers were stolen, or how many customers were affected. But it noted that it does not collect PINs in its stores.
(Read more: 5 lessons learned from the Target security breach)
The company only publicly acknowledged the data breach last Friday after facing inquiries from a journalist specializing in computer security, Brian Krebs.
But Neiman had told credit card companies around Christmas, in an industry phone call, that it had evidence that creditcards used at Neiman Marcus were being used to make fraudulent purchases,people who were briefed on those phone calls said.
Neiman has faced criticism for not telling customers about the breach sooner. Target informed its customers about its attack within a few days of learning about it for the first time, pushed also by inquiries from Mr. Krebs. Target's acknowledgment came right before the final holiday shopping weekend, and put a significant dent in Target's sales in the final seasonal rush.