The cyber crime is believed to be led by Aquabox, and dozens of botnet operators remain at large and the authorities are working to uncover their identities. Boscovich said he suspects Aquabox is in Eastern Europe.

The botnets hosted from "command and control" servers at data hosting centres around the world, were used to steal from hundreds of financial institutions, according to Microsoft, which cited the reason to get permission from courts to shut down the infecting servers. However the shocking thing is that the data centre operators were unaware about botnets running on their servers.

The group targeted firms of all sizes, from tiny credit unions to global banks such as Bank of America, Credit Suisse, HSBC and Royal Bank of Canada.

Citadel is one of the biggest botnets in operation today, and Microsoft said that it is giving out pirated versions of the Windows operating system.

The FBI, declined to comment on its progress in its investigation of Citadel, has said it is working closely with Europol and other overseas authorities to capture the unknown criminals.

The Citadel software disables anti-virus programs on infected PCs so they cannot detect malicious software. It surfaced in early 2012 and is sold over the Internet in kits that cost $2,400 or more.

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