Experts are beginning to think that the thwarted hacker robbery of a planned $423 million from a Japanese bank in London likely involved an insider, British newspapers reported Friday.

The criminals are thought to have tried to access the network of Sumitomo Mitsui through a keylogger, a program that surreptitiously records every keystroke to steal usernames and passwords.

According to reports in the Evening Standard, a security source claimed that one of the gang members had been planted inside the bank to download the keylogger and install it on a PC.

"Why go through the front door of a major financial institution, where the firewall is toughest, when you can simply sneak round the back?" Jonathan Mepsted, U.K. director of security firm Fortinet, told the Guardian, another British newspaper.

Other experts theorized that that insider might well turn out to be a former employee, or even someone on the cleaning staff.

"Firms spend vast sums on technology but invariably take on some employees and temps on low wages with minimum background checks," intrusion detection expert Emlyn Everitt was quoted by the Guardian. "You're only as strong as your weakest human link."

It would have been relatively easy, added other experts, for some on the clearing crew to add a keylogger to a system, or even plant a dongle-style keystroke interceptor between a keyboard and the computer.

Thursday, the National Hi-Tech Crime Unit (NHTCU), Great Britain's national cyber-cops, broke up the scheme, and helped arrest a 32-year-old Israeli who was poised to transfer 13.9 million pounds ($26.8 million) into his account there. All told, the gang was planning to transfer the $423 million to 10 different bank accounts, said police.