Police suspect the cash was withdrawn simultaneously using counterfeit credit cards containing account information leaked from a South African bank.
Investigators now suspect a large-scale international crime organization was involved. Police investigators will work with South African authorities through Interpol to probe the incident, including how credit card information was leaked, the sources said.
The theft took place on the morning of May 15 in Tokyo and 16 other prefectures, and police believe more than 100 people might have worked together to execute the grand theft.
In each of the approximately 14,000 transactions, the maximum amount of ¥100,000 was withdrawn from Seven Bank ATMs using the fake credit cards, according to the sources.
ATM transaction data suggests that information from 1,600 credit cards issued by a South African bank was used, the sources said.
Police are now checking images taken by security cameras in the stores to identify those who withdrew the money.
In 2012 and 2013, a total of ¥4.5 billion was withdrawn with forged credit cards from financial institutions in more than 20 countries, including Japan.
Japanese police have put suspects belonging to a Malaysian group on an international wanted list.
Convenience store ATM heist sees ¥1.4 billion stolen in two hours across Japan | The Japan Times