Am pretty curious what kind of "skeleton key" ATM card they were using to access all of the ATM's as different users. Lest they were posed as repair people and stood there with a laptop hooked in.
Generally in these cases spoofed cards + access to back end system. They just look like ordinary customers from the ATM perspective.
From what I've seen the attackers pay organized teams of people to do the withdrawals that wouldn't have a problem getting caught by the cops for a percentage. Load a stack of blank cards, then send out the squad.