"It was pretty ingenious," Pace University computer science professor Darren Hayes said on Friday.
On the creative side of the heist, a small team of highly skilled hackers penetrated bank systems, erased withdrawal limits on prepaid debit cards and stole account numbers.
On the crude end, criminals used handheld devices to change the information on the magnetic strips of old hotel key cards, used credit cards and depleted debit cards.
Seven people were arrested in the US, accused of operating the New York cell of what prosecutors said was a network that carried out thefts at ATMs in 27 countries.
Law enforcement agencies from more than a dozen nations were involved in the investigation, which was being led by the Secret Service.
Here's how it worked:
First, the hackers, quite possibly insiders, broke into computer records at a few credit card processing companies, first in India and then the US This has happened before, but here's what was new: They didn't just take information. They actually raised the limit on prepaid debit cards kept in reserves at two large banks.
"It's pretty scary if you think about it. They changed the account balances. That's like the holy grail for a thief," said Chris Wysopal, co-founder of security company Veracode.
The next step was technically simpler, almost an arts-and-crafts activity.
Crime ring members in 27 countries ran used plastic cards, just about anything with a standard magnetic strip, through handheld magnetic stripe encoders, widely available online for less than $300. Those devices allow users to change information on magnetic stripes or to write new cards with a simple swipe.
In this case, the stripes were rewritten with information from the hackers. That allowed the thieves to turn the cards into gold, instantly transforming them into prepaid debit cards with unlimited amounts of money stored on them.
Finally it was time for action.
On two pre-arranged days - once in December and again in February - criminals loaded with the lucrative debit cards and PIN numbers, headed into city streets around the world, racing from one ATM to the next, often taking out the maximum the cash machine would allow in a single transaction: $800.
In December, they worked for about 2½ hours, reaping $5m worldwide in about 4,500 transactions. Two months later, apparently buoyed by their success, they hit the ATMs for 10 hours straight, collecting $40m in 36 000 transactions.
The New York money runners made off with $2.8m, according to the indictment, a fraction of the total amount yielded by the heist.
Players kept a cut but sent the bulk of the money back to the masterminds through wire transfers and sometimes in person, prosecutors said.