Bay City News -- A Lucky supermarket in San Leandro is the 24th Northern California store victimized by a credit and debit card-skimming operation, Save Mart Supermarkets has announced.

More than 500 Lucky store customers have contacted the company regarding fraudulent or attempted fraudulent activity, Save Mart Supermarkets said in a customer alert on its website. Save Mart Supermarkets operates the Lucky chain. In a consumer advisory issued on Nov. 23, the company said it had learned that card-reading machines in the self-checkout lanes at 20 stores contained data skimming devices. The first tampered card reader was discovered during a regular inspection, the company said.

Three more stores were added to the list Monday, and the San Leandro store at 1300 Fairmont Drive was added Wednesday. The 24 affected stores are located all over the Bay Area.

"Every customer should continue checking their accounts regularly as a precaution if they used a checkout terminal in October or November," the customer advisory stated. Local, state and federal agencies are involved in the investigation, and the U.S. Secret Service is examining the tampered card readers, company officials said.

Save Mart spokeswoman Alicia Rockwell said the Secret Service called the skimming devices "extremely sophisticated."

Only one self-checkout card reader in each of the 24 stores was compromised, and all the affected card readers were replaced as of Nov. 23, according to the company.