Exclusive The story of the investigation into the failed multi-million pound cyberheist at Sumitomo Bank can finally be told, following the recent conviction and sentencing of its perpetrators.

The audacious Mission Impossible-style scam, which brought a pair of hackers and a bent insider together with other fraudsters, sought to spirit away £229m ($423m) from corporate accounts held at the London offices of the Japanese bank Sumitomo Mitsui in October 2004.

The conspiracy narrowly failed and no money was ever lost. Had it succeeded, the amount stolen would have dwarfed the £26m in gold bullion taken in the November 1983 Brinks Mat robbery and the £40m Securitas job in 2006.

Tech-savvy cybercrooks were smuggled into Sumitomo's offices in September 2004, where they used commercial keystroke-logging software - not spyware or specialist hardware, as initially and widely reported - to capture usernames and passwords needed to make Swift bank transfers.

Marc Kirby led the investigation

These stolen login credentials were used in an unsuccessful attempt to transfer money to ten overseas accounts under the control of fraudsters a month later.

Repeated attempts to transfer funds to accounts in Spain, Dubai, Hong Kong and Singapore failed because of errors in completing one of the fields in the Swift system used to make transfers. But for this failure, accounts held by firms including Toshiba International, Nomura Asset Management, Mitsui OSK Lines and Sumitomo Chemical would have been plundered.

The score

Returning to work after the weekend break, Sumitomo staff noticed that PCs had been tampered with. Executives reported the crime after receiving notice of the failed transfers, sparking a painstaking two-year police investigation.

The investigation was led by Marc Kirby, retired former detective inspector at the National Hi-Tech Crime Unit and later an officer at SOCA, who saw the investigation through from the first phone call from the bank and spoke exclusively to The Register.

"We quickly established something untoward had happened when we checked the CCTV footage and discovered tampering," Kirby told El Reg. "The sensitivity had been altered, turned down, so that the cameras didn't record what was happening on the trading floor."

Police quickly recognised the heist as an inside job, seizing building entry records and CCTV footage from street cameras. Security supervisor Kevin O'Donoghue, 34, became a key suspect once it was realised that computers were tampered with over the weekend.

O'Donoghue repeatedly smuggled two men - later identified as the hackers involved in the plot - into Sumitomo's London office. When challenged by other workers, O'Donoghue claimed the pair were there only for a card game.