Images released by police show an ATM skimmer that was placed on an ATM at the First Midwest Bank, 241 East Deerpath Road in Lake Forest. | Provided Photo.

Police are warning the public after a bank technician found a skimming device covertly installed on an ATM in Lake Forest.

The Lake Forest Police Department said that the bank technician was servicing an outdoor ATM at First Midwest Bank, 241 East Deerpath Road, on Tuesday.

The technician located an internal bank card skimmer — called a ‘shimmer’ — that had been covertly installed inside the ATM’s card reader slot.

Police say the ATM had been serviced four days prior on February 12, so the device had likely only been on the machine for a few days.

Anyone who used the drive-thru ATM is asked to monitor their account carefully and report any false charges to their bank immediately.

Lake Forest police say they are working with First Midwest Bank to investigate the incident. Police are looking into video surveillance footage to help identify the suspect.

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“[We] are very thankful to their alert employee who spotted this device and reported it as soon as possible!” police said in a statement.

According to the FBI, skimming occurs when devices illegally installed on ATMs or credit card terminals capture data or record pinholders’ PINs.

Criminals use the data to create fake debit or credit cards and then steal from victims’ accounts. It is estimated that skimming costs financial institutions and consumers more than $1 billion a year, the FBI said.