As of Thursday, more than 280 people said they became victims of fraud after visiting the gas station, with losses totaling more than $82,000.
Most of the crimes involved a single fraudulent transaction for an amount less than $200, police said. Most of the multiple-transaction cases involved ATM and retail transactions, police said. The largest single loss was nearly $3,800.
"While debit and credit fraud is not an uncommon crime, the nature of this crime and the number of people it's affected is very unusual for the city of Sierra Madre," said Mayor Joseph Mosca.
Investigators from the U.S. Secret Service have joined Sierra Madre Police in working to solve the case.
Meantime, the EVG gas station has been closed by the owner. Police believe the owner of the gas station may have also used stolen money to purchase his property.
Police urged people who used credit or debit cards at the station to check their accounts dating back to June and to notify police of any suspicious activity. Anyone with information on the case was urged to call Sierra Madre Police at (626) 355-1414 or 355-5247.
City News Service contributed to this report.