The number of Lucky supermarket customers suspecting foul play as a result of a debit and credit card-skimming scheme has leapt to more than 500, and four more stores were found to have been targeted.

Lucky supermarkets in Daly City, Foster City, Millbrae, Redwood City, and San Carlos are among 24 Northern California stores hit in the swindle.

When a skimming device was discovered during a routine inspection in November, the company said no customer account had actually been penetrated.

But as news surfaced of hundreds of instances of thefts and attempted thefts, up to 2,000 patrons a day have been flooding company phone lines, says a customer alert posted online by Save Mart Supermarkets, the parent company for Lucky.

Media reports claiming more than 1,000 have been victimized are inaccurate, said Super Mart spokeswoman Alicia Rockwell in an email message Friday evening.

Investigators discovered skimmers targeted three more stores than originally thought, and a San Leandro store was added Wednesday. The 24 markets span the Bay Area.

A physical fact of the electronic age, skimming violates state and federal laws, and local, state and federal agencies are involved in the investigation. The U.S. Secret Service, which investigates credit card fraud, counterfeiting and other financial crimes, is studying the doctored card readers, company officials said.

Bay City News Service reported that after an inspection, Secret Service agents found the hardware to be "extremely sophisticated."

So much so that the thieves could retrieve information from the hardware remotely and avoid the risk of returning to the stores, Save Mart Chief Financial Officer Stephen Ackerman told The Associated Press.

A skimmer is a sleeve or an insert that can be easily installed without attracting attention, said Joan McNabb, chief of the California Office of Privacy Protection. A skimmer will read all the account information stored electronically on the magnetic stripe of a debit card, and record PINs as they are entered on the keypad.

“It's not an ultra-complicated thing to install,” McNabb said. "Plus, (the perpetrators) were in the self-checkout, so they were more likely to be unobserved."

Card skimming is not restricted to the Lucky chain or to grocery stores at all, McNabb said. Consumers have been defrauded by skimmers at gas stations, ATMs, restaurants. And thefts of this sort are more likely to take place on weekends when financial institutions are closed and businesses operate with limited staff.

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. Customers with compromised account statements are encouraged to contact the Lucky Customer Support Center at (800) 692-5710.

What to do if your card was skimmed:

• Contact your bank and tell them you want to close the account (get new one with new number)
• If you encounter resistance to removing unauthorized charges, dispute it in writing
• If the bank asks for it, get a police report of identity theft
• Download the California Office of Privacy Protection information sheet

Source: California Office of Privacy Protection

Lucky Supermarkets hit by the skimming operation by county include the following:

Alameda County: Alameda, Union City, Fremont, Hayward

Contra Costa County: El Cerrito, Pinole

Marin: Novato

San Francisco

San Mateo County: Daly City, Foster City, Millbrae, Redwood City, San Carlos

Santa Clara: San Jose, Santa Clara, Milpitas, Mountain View, Sunnyvale

Sonoma: Petaluma