Faced with the equivalent of a cyber-crime tidal wave, the FBI has at last begun to grapple with such arcane computer mischief as the deployment of Trojan horses — destructive data viruses. Agents are learning how to recognise the infamous salami attack — programmes that capture information in barely detectable slices — and how to defuse a logic bomb.

According to one knowlegdeable source, another hacker brags that he recently found a way to get into Citibank’s computers. For three months he says he quietly skimmed off a penny or so from each account. Once he had $ 200,000, he quit. Citibank says it has no evidence of this incident and we cannot confirm the hacker’s story. But, says computer crime expert Donn Parker of consultants SRI International: "Such a salami attack is definitely possible, especially for an insider."

The "salami attack" executes barely noticeable small acts, such as shaving a penny from thousands of accounts.