London (AP) — 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 from its coffers, police said Friday, an indication of the potential for electronic crime to tear huge chunks off financial institutions’ balance sheets.

London 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 center.

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.”

Writing on the blog of Internet security firm Sophos, John Hawes said it wasn’t clear how much damage the would-be robbers might have done “even with access to a workstation.”

“If the systems were well controlled, secured and monitored, there should still have been plenty of obstacles to overcome before they could find their way into sensitive parts of the network, and move virtual cash out of the bank’s systems,” he said.

Police said they took the attempted robbery very seriously. In their statement, Det. Insp. Mark Raymond described it as a “sophisticated plot that could have led to the loss of a very large amount of money from the bank.” The force put the potential losses in the millions of pounds — although it stressed that no money was ever withdrawn.

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

The scale of the potential theft is another reminder of the huge 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.