The extraordinary list of nightmare disasters, most triggered by terrorists, is being used by the Department of Homeland Security to concentrate its resources in the areas of most likely attack.

The list, released mistakenly on to a website in Hawaii, shows where security officials believe the United States to be most vulnerable. It also includes a detailed breakdown of the expected casualties and economic costs that such attacks — and some natural disasters — would exact.

One of the most deadly of the 15 scenarios is a flu pandemic, which begins in southern China and spreads within months to four leading American cities, claiming the lives of 87,000 and putting 300,000 in hospital, the plans estimate.

A ten-kilotonne nuclear bomb driven by van into a big city before being detonated would be the most expensive, costing hundreds of billions of dollars, according to the planners. Casualties from such an explosion “could vary widely”, they say.

The Homeland Security Department, set up in the aftermath of the September 11, 2001, attacks, provoked widespread criticism because of the way it distributed money in its first years. It allocated cash state by state, so that small towns in the middle of Wyoming were equipped with brand new equipment for hazardous materials units, while target cities such as New York and Washington received no extra cash.

Michael Chertoff, the new Homeland Security Secretary, has promised to overhaul the process. The nightmare scenarios are part of a plan to ensure that cash for emergency planning is allocated according to likely need.

The list of scenarios concentrates heavily on chemical and biological attacks. It envisages terrorists spraying anthrax with aerosols from a van as they drive through three cities. They would be able to hit another two shortly afterwards before authorities were able to grasp what was happening.

Such an attack would leave 13,000 dead and cost billions of dollars, according to National Planning Scenarios, the document. By contrast, terrorists using a small aircraft to spray chemical blister agent over a packed college football stadium would leave 150 dead and 70,000 taken to hospital, costing $500 million (£261 million).

The release of pneumonic plague into an airport washroom, a sports arena and a train station in a big city, spreading rapidly, would leave 2,500 dead and 7,000 injured and cost millions of dollars.

If terrorists released sarin gas into the ventilation systems of three large office buildings, it would kill 6,000 and cost $300 million. Several scenarios envisage terrorists using explosives to trigger wider disasters. Blowing up a storage tank of chlorine gas and releasing a large quantity downwind would leave 17,500 dead, 10,000 severely injured and 100,000 taken to hospital.

Clark Ervin, a former Homeland Security inspector-general, denied that the list helped terrorists by revealing the nation’s vulnerabilities. “The terrorists know what their objectives are. They know what the vulnerabilities are,” he said. The report was likely to deter attacks in these areas because it showed that the US was on its guard, he said. “And if attacks occur it’s likely to minimise the damage.”

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