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Detective Superintendent Terry Wilson, head of the Metropolitan Police's central e-crime unit, said: “This was a highly-organised criminal network with each individual filling a specific role.
"All criminal networks have a head and we very much believe we have now apprehended our ‘Mr Big’ as part of this operation.
"We believe this has put a major dent in this alleged criminal network’s ability to commit cyber-crime in this country. We believe this was a top-tier criminal network."
Police believe the gang that targeted Barclays had a "control room" in Seymour Place, Marylebone, central London.
The Barclays theft is being dealt with by the same investigation team that is dealing with the Santander attempted robbery.
A gang is alleged to have tried to use the KVM gadget to take control of all the computers at a branch of Santander in Surrey Quays shopping centre in south east London.
The attempt failed and the Spanish bank said they were unable to steal any money.
- A keyboard, video and mouse switch - known as a KVM - is a hardware device which is used legitimately in business to allow users to work remotely on their computer systems.
- It allows a user to control multiple computers from one keyboard, video monitor and mouse.
Cash, jewellery, drugs, thousands of credit cards and personal data have been seized in searches at addresses in Westminster, Newham, Camden, Brent and Essex.
Detectives described one of the addresses in central London as the fraudsters' control centre.
A KVM device, which allows users to work remotely on a system, and 3G router were found connected to a computer at the Swiss Cottage branch in north London.
A male purporting to be an IT engineer allegedly accessed the branch the day before the theft and attached the KVM device, Scotland Yard said.
This allegedly allowed a group to remotely transfer money to a predetermined bank account.
Barclays Banks reported a £1.3 million loss from its Swiss Cottage branch in April 2013 and an investigation was launched, Scotland Yard said.
The men, aged between 24 and 47, were arrested in connection with an allegation of conspiracy to steal from Barclays Bank and conspiracy to defraud UK banks, Scotland Yard said.
They remain in custody at a London police station.
Eight men have been arrested in connection with a £1.3 million theft by a gang who took control of a Barclays Bank branch computer system, Scotland Yard said.
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Eight men have been arrested over a £1.3 million theft by a gang who took control of a Barclays Bank branch computer system.