Fraudsters drained £1.3 million from Barclays accounts by remotely accessing a branch computer system.
The police linked the theft at Barclays ( to another attempt to steal money from a )Santander ( branch in London, which led to the arrest of 12 men last week. )
"The size and sophistication of the operation has shocked us and we're actually having to draft in more resources to deal with the amount of evidence that we've recovered," said detective superintendent Terry Wilson.
"This was a highly-organized criminal network," said Wilson. "All criminal networks have a head and we very much believe we have now apprehended our 'Mr Big' as part of this operation."
The story dates back at least to early April when a man pretending to be an IT engineer gained access to computers at a Barclays branch in London after convincing staff he was there to fix a fault.
Police say he installed a keyboard, video and mouse (KVM) switch that allowed him to gain remote access to the bank's computer system.
Police made the arrests linked to the Barclays plot on Thursday and Friday, and traced the fraudsters' "control center" to a home in central London.
They are still in the process of seizing property, cash, jewelry, drugs, thousands of credit cards and personal data in relation to the case. The eight men were arrested on suspicion of stealing and defrauding banks.
Of the 12 men arrested last week, four were charged with conspiracy to steal, while the others were bailed but face further investigation.
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