In each case, police said, a keyboard video mouse device was attached to the bank's computer system, allowing money to be remotely transferred to accounts under the thieves' control, The Guardian reported Friday.
Eight people were arrested Thursday in the $2.1 million theft from a Barclays bank branch in North London April 5.
Investigators said a man posing as a computer engineer had attached the mouse device to the bank's computer the day before.
Another 12 people were arrested last week in what police said was a nearly identical plot to steal from a Santander bank branch in southeast London. Metropolitan police discovered the device before it was activated.
The suspects in the Barclays theft "are significant players within a sophisticated and determined organized criminal network who used considerable technical abilities and traditional criminal know-how to infiltrate and exploit secure banking systems," said Detective Inspector Mark Raymond of the central e-crime unit.
Investigators said they found cash, jewelry, drugs, thousands of credit cards and personal data in searches of buildings connected to the suspects.
Police said a large amount of the money stolen from Barclays had been recovered.
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