A new report has claimed up to 100 banks and financial institutions worldwide have been attacked in an "unprecedented cyber robbery".
Computer security firm Kaspersky Lab estimates $1bn (£648m) has been stolen in the attacks, which it says started in 2013 and are still active.
It said a cybercriminal gang with members from Russia, Ukraine and China is responsible.
Kaspersky said it worked with Interpol and Europol on the investigation, and said the attacks had taken place in 30 countries including financial firms in Russia, US, Germany, China, Ukraine and Canada.
Sanjay Virmani, director of Interpol's digital crime centre said: "These attacks again underline the fact that criminals will exploit any vulnerability in any system."
The gang, which Kaspersky dubbed Carbanak, used computer viruses to infect company networks with malware, including video surveillance, enabling it to see and record everything that happened on staff's screens.
In some cases it was able to transfer money from the banks' accounts to their own, or they were even able to tell cash machines to dispense cash at a pre-determined time of day.
Kaspersky believes on average each bank robbery took between two and four months, with up to $10m stolen each time.
Kaspersky Lab's principle security researcher Sergey Golovanov said: "It was a very slick and professional cyber robbery."
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