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tial destruction of the 911 system was now the real nightmare scenario.

Just seven months later, on the public holiday in honour of Dr Martin Luther King, the most sophisticated telephone system in the world went down for nine hours. At 2.25 p.m. on 15 January 1990 the nation-wide network operated by AT&T, North America's biggest phone company, providing national and international long-distance service, was hit by a computer failure. For the duration of the breakdown, the only voice responding to millions of long-distance callers was a recorded message: 'All services are busy - please try again later.'

It was estimated that by early afternoon as many as half the long-distance calls being dialled in every major city were blocked. Some 20 million calls were affected, causing chaos in many businesses, especially those like airlines, car rental companies and hotels which relied on free 1-800 numbers. It was the most serious failure since the introduction of computer-based phone systems thirty years earlier.

Robert E. Allen, AT&T Chairman, emerged the following day to explain that 'preliminary indications are that a software problem occurred, which spread rapidly through the network'. Another spokesman said that while a failure in the software systems was probably to blame, a computer bomb could not be ruled out. The problem had been centred in what was called a 'signal node', a computer or switch attached to the network. The errant system 'had told switches it was unable to receive calls, and this had a domino effect on other switches,' according to AT&T. The effect was not dissimilar to the endless loop, which causes all incoming calls to circle idly around the switch.

Software problems are not uncommon, but few have such spectacular effects. And coming so soon after the computer bomb threat, rumours flourished that AT&T had been hit by hackers. In the course of researching this book, the authors were told more than once that the AT&T failure had been caused by a computer bomb. One source even claimed he could identify the culprit. The rumours continue to circulate, as they do about everything in the computer underworld.

In fact there is absolutely no proof that it was a computer bomb,

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