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It's a new kind of bank robbery: hackers break into ATM machines through holes in the computer software, stealing money from a variety of bank accounts.

It's something you probably take for granted that when you use an ATM machine and enter your personal PIN code, that secret number stays between you and your bank. Think again.

We're at a tipping point where this is actually becoming a serious problem.

It happened most recently at 7-Eleven stores across the country. Hackers broke into Citibank's networked ATM machines, snatched people's PIN numbers and stole some $2 million from a range of bank accounts.

ATM crime has become more and more sophisticated. Ah, you see organized crime rings for example, uh, that, you know, they are willing to sometimes invest, you know, large sums of money upfront, uh, in order to, uh, compromise, you know, in order to compromise an account or accounts.

ATM trade spokesman Kurt Helwig says it was an isolated incident. Still the question: how they do it? Sixty percent of ATM machines are owned by private independent companies. Prosecutors are investigating the possibility that the thieves hacked into the private ATM computer network owned by Cardtronics, stealing the PIN codes during transmission before they ever reach the bank.

Citibank tells CNN that the situation has been resolved; there have been no fraudulent transactions since March. Three people believed to be at the center of the theft have been indicted. And Citibank says thy've repaid anyone whose accounts may have been compromised.

In the past, ATMs have been compromised by phony card scanners or cameras that record people punching in their PIN numbers. This raises the stakes because hackers can steal an unlimited number of PIN codes electronically. The ATM industry stresses this was an isolated incident affecting only a tiny percentage of the one billion yearly ATM transactions.

For CNNMoney. com, Deborah Feyerick.