Detectives investigating the cloning of bank cards at a petrol station in Wirral believe a camera had been fitted to record the pin details of customers.
Hundreds of customers of the petrol station in Greasby complained that the details of their bank cards had been taken and used in London and Thailand.
A pin-hole camera was found fitted to the roof of the garage with the ability to spy on customers' pin numbers.
Officers are now investigating allegations of fraud.
It is believed the camera could have been fitted with the help of someone with legitimate access to the garage.
Police are warning people to use their hands to cover the display when typing in their pins to prevent offenders from seeing confidential details.