Napa police have found the machines that process debit and credit card purchases at Hancock Fabrics are the common thread connecting dozens of recent cases of fraud.

Hancock Fabrics on Imola Avenue has replaced its machines, and the investigation into the thefts continues, said Lt. Brian McGovern, spokesman for the Napa Police Department. Meanwhile, the number of reports Napa area residents have made to the department has risen to 50, he said.

Victims noticed strange purchases or ATM withdrawals on their accounts from locations around the Bay Area in the past several days. The thefts are not linked to any particular bank, McGovern said.

Additionally, the Sacramento County Sheriff’s Department informed Napa police of at least five card swipe machines tampered with at other Hancock Fabric locations, McGovern said.

The local Hancock Fabric store referred a call to its corporate office, where officials did not return calls for comment.

There’s no foolproof way to avoid this kind of theft when using debit and credit cards other than the common-sense precautions, said Patricia Trimble, branch manager for US Bank in Napa.

“I wish I had a secret,” she said. “If I did, I would be a bazillionaire.”

Trimble said the best advice is to keep cards with you and guard the keypad when entering personal identification number, she said. Debit card transactions that require a PIN are more secure than credit card transactions, Trimble said.

She advised people to check their accounts daily for any suspicious activity.

“If you see something wrong, don’t wait. Speak now,” she said.

US Bank and others have fraud departments that call customers when suspicious charges — large transactions, those that occur in rapid sequence or those that take place far from the cardholder’s home — appear on their accounts.

If a customer suspects a problem, he or she should call the branch that opened the account or the bank’s 1-800 number, Trimble said.

It’s becoming more common for banks to simply close accounts and issue new cards when suspicious activity is reported.

“It’s just paperwork, and we don’t mind doing it because it protects our people,” she said.

McGovern advised residents who have been to Hancock Fabrics within the past several months to review their accounts carefully.

Residents who find suspicious activity on their accounts can make reports on the Napa Police Department’s Web site. Click on the “Online Crime Report” heading on the left-hand side of the home page, then on the “Napa Police Online Crime Reports system” link in the center of the page. Finally, scroll down and click on “theft.”

If you have made a purchase at Hancock Fabrics and haven’t seen any theft, the safest route is to notify your bank, close the account, open a new one and change your PIN, McGovern said.