Eight men have been arrested in connection with a £1.3m theft by a gang who took control of a Barclays Bank computer.
The money was transferred from the branch in Swiss Cottage in north London in April, a Met Police spokesman said.
Searches are being carried out at addresses across London where property including cash, jewellery, drugs and credit cards has been seized.
The raid is being linked to an attempt to steal from Santander last week.
Four men have appeared in court charged with attempting to take control of computers at a Santander branch in Surrey Quays, south-east London.
Det Supt Terry Wilson said the Barclays investigation was being carried out by the same police team.
However he said the latest arrests "are a different level, it's really the top tier of this criminal network that have been arrested".
He added: "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."
The men, aged between 24 and 47, were arrested on Thursday and Friday.
Following the report of the theft, police found a "keyboard video mouse" (KVM) switch attached to one of the branch's computers.
It had had been placed there by a man purporting to be an IT engineer the day before the theft on 5 April.
Infiltrate and exploit
A KVM switch, which has a 3G router attached, allows a user to control multiple computers. This enabled the gang to remotely transfer funds to other back accounts.
Barclays said it was able to recover a "significant amount" of the stolen money.
One central London premises which has been searched was described by detectives as the "control" centre for the fraudsters.
"Those responsible are significant players within a sophisticated and determined organised criminal network, who used considerable technical abilities and traditional criminal know-how to infiltrate and exploit secure banking systems," Det Insp Mark Raymond of the Met's Central e-Crime Unit said.
Alex Grant from Barclays said: "Barclays has no higher priority than the protection and security of our customers against the actions of would-be fraudsters.
"We identified the fraud and acted swiftly to recover funds on the same day. We can confirm that no customers suffered financial loss as a result of this action."