A bank customer found a skimmer as he used an ATM, one of several found in Bal Harbour.
"I juggled it a little bit, and I started to take it off and I could feel that there were adhesive there. I was hoping that (I) wasn't breaking the machine," said Sean Clancy. "I was a little shocked."
A police sergeant also found one on another ATM.
The skimmer, combined with a pinhole camera, helps thieves steal debit card numbers.
"You got to have quite a bit of computer knowledge, electronic knowledge," said Detective Paul Eppler.
Several online companies sell the devices.
"This is a big problem," said Diana Goudie, a branch manager at JGB Bank. "The banks would be then the ones that lose the money because we have to give it to the client."
Federal authorities are assisting in the investigation.