A daring attempt to graft a rogue piece of hardware onto a computer at a London branch of Spanish bank Santander could have drained millions of pounds (dollars) from its coffers, police said Friday, an indication of the potential for electronic crime to tear huge chunks off financial institutions’ balance sheets.

London’s Metropolitan Police and Santander said in a joint statement that 12 suspects were arrested Thursday following an attempt by a bogus maintenance engineer to install a keyboard-video-mouse – a device typically used to control several computers at once – onto one of the bank’s computers at a branch located in a south London shopping centre.

Few other technical details were released, but the statement said that the hardware would have allowed the transmission of the entire computer’s desktop and “allowed the suspects to take control of the bank’s computer remotely.”

“This was a sophisticated plot that could have led to the loss of a very large amount of money from the bank,” Detective Inspector Mark Raymond said in the statement. No money was ever withdrawn, but the force put the potential losses in the millions of pounds.

It’s not clear from the statement whether the person masquerading as an engineer was arrested at the scene or when he tried to install the rogue software. Police said that all but one of the 12 suspects, ranging in age from 23 to 50, were apprehended in the same west London neighbourhood.

The scale of the potential theft is another reminder of the massive amounts that can be stolen by tech-savvy criminals. U.S. Investigators say that one gang operating across 27 countries recently managed to steal $45 million in two separate sprees after compromising payment systems used by two Middle Eastern banks.

The suspects in the latest heist remain in custody. Police said searches were being carried out in six different locations in the greater London area. Santander said none of its staff were involved in the attempted heist.