Taiwan's Investigation Bureau said that when information security personnel looked to see how First Bank's network was hacked into and how the ATMs were controlled, they found there had been irregularities in the connection between the voice recording server in London and the bank's internal network and the ATM machines in Taiwan.
In analyzing the connections, the technicians discovered malware and content that should not have been there, and they concluded that the server was the likely network endpoint exploited by the hackers.
They said the bureau indicated that the initial hack took place on July 4 and was tested on ATM machines on July 9.
The bureau said that because First Bank's computer system is a closed network, it suspected that an international ring had hacked into the computer system of the bank's London branch and then obtained the account number of the ATM computer system's administrator, giving the ring access to the internal network.
To check its suspicions, the bureau summoned staff from the bank's London branch back to Taiwan on Friday to cooperate in the investigation.
Staff from the bank's London branch and headquarters as well as personnel from the manufacturer of the compromised ATMs, Wincor Nixdorf, were questioned by the bureau on Monday.
Meanwhile, First Bank Chairman Tsai Ching-nain said Monday that 51 of the bank's 438 Wincor PC1500 ATMs were hacked into on July 9 and 10, the weekend when the heist occurred.
Tsai said the company has decided that the PC1500 ATM will be phased out and will no longer be online, and indicated that deputy managers and other executives will be disciplined because of the heist.
He said he himself wanted to know why the international ring targeted the bank, and he hoped the Investigation Bureau could use its high-tech tools to find the answer.
Tsai also wanted the experience to be shared with banking counterparts in the country, hoping that this will be "the last (hacking) case in Taiwan."
An international ring used malware to have several First Bank ATM machines freely dispense money without the need of a bank card to access the machines. They stole more than NT$83 million.
Thirteen of the 16 suspects fled Taiwan, but police were able to arrest three suspects and retrieve about NT$60 million of the stolen money on Sunday. (By Tien Yu-ping and Lilian Wu)