Toronto police Staff Inspector Bruce Evans told a news conference the devices read either the debit or credit card data while a tiny pinhole camera records the users punching in their security codes, CTV News reported.
That information can be used to either make false credit cards for purchases or cash withdrawals.
"We believe that the hospitals are being targeted because of the high volume of pedestrian traffic and the fact that the ATM machines are located within open common areas," Evans said Tuesday.
No arrests were reported and the number of victims or their losses weren't disclosed.
Detective Ian Nichol said the investigation is far from over.
"It would be foolish of us to presume that we got every single one of them," he said. "In fact, I believe that we didn't."