Secret Service warning banks about "jackpotting"

The Secret Service is warning banks about "jackpotting."

This is when thieves make ATMs spit out cash like slot machines at 40 bills every 23 seconds.

The Secret Service said they've seen about a dozen cases across the country.

"What we're finding is the attacks really started in a coordinated effort in December and well north of a million dollars has been taken," Supervisory Special Agent Matt O'Neill said.

Here's what the Secret Service told ABC News about how jackpotting works:

Fraudsters pose as ATM technicians, even wearing uniforms and access the teller machine. They open it using a generic key that the Secret Service says is readily available for purchase on the internet. Once inside, they use a technical means -- installing a laptop computer and a cellphone into the machine -- to be able to remotely take over the machine and force it to discharge money. But to avoid detection, the bogus technician does not typically take the cash, That's left to a second co-conspirator.

The second co-conspirator, a "money mule," then goes to the compromised machine and calls the phony technician who initiates a withdrawal sequence remotely. "It runs until it is empty or the person standing at the ATM alerts the controller of the ATM to stop the withdrawal sequence because either law enforcement is nearby or for whatever reason they get spooked and want to leave the scene," O'Neill told ABC News.

The technician often returns to the empty machine to retrieve the the laptop and cellphone, putting the ATM machine back like he was never there.

Two of the world's largest ATM markers have sent out alerts warning banks and other financial firms after the first jackpotting case was reported last week.

In the past, thieves have used skimming devices on ATMs to steal debit card information or swap out the machine's hard drive.