Target America: the simple device for causing an explosion of panic

After the steady drip-drip of revelations about the parlous state of America's domestic security over the past month, along comes a new nightmare: the startlingly simple recipe for detonating a "dirty bomb". As we learnt yesterday, all it takes is small quantity, not necessarily more than a teacup, of a radioactive isotope such as caesium-137 or strontium-90. Both are used for cancer treatments in hospitals and are relatively easy to get hold of.

After the steady drip-drip of revelations about the parlous state of America's domestic security over the past month, along comes a new nightmare: the startlingly simple recipe for detonating a "dirty bomb". As we learnt yesterday, all it takes is small quantity, not necessarily more than a teacup, of a radioactive isotope such as caesium-137 or strontium-90. Both are used for cancer treatments in hospitals and are relatively easy to get hold of.

The radioactive material can be packed into a conventional explosive ­ anything from a nail bomb to the fertiliser-based device that devastated the Oklahoma City federal building seven years ago ­ or, even more simply, loaded into an aerosol spraying device. And then it's just a matter of pressing a button.

The result would be haphazard and hard to measure. The number of dead and injured would depend largely on the force of the conventional explosion rather than the spread of radioactive contamination, which could be relatively modest. But one thing is almost certain: it would create panic on a gigantic scale.

We have to be careful about our terms here. A dirty bomb is not a nuclear bomb, not even close; it is a crude system for dispersing radioactivity. Certainly, if the radioactive material used was weapons-grade plutonium, or spent nuclear fuel, it could cause horrific casualties, especially over time. The more immediate threat, however, is something less dramatic, something that would be more devastating psychologically and symbolically than in terms of the raw human cost.

Imagine an explosion in the centre of Washington, say. First indications from the Justice Department suggest that could have been the intention of Abdullah al-Muhajir, the dirty bomb plot suspect whose arrest was announced yesterday.

Given the massive security presence in the capital since 11 September, an important building could probably not be taken out along the lines of the Oklahoma City bombing in 1995. But just imagine the effects of even a small explosion somewhere in the vicinity of the Capitol building, or the White House.

Whole city blocks would be evacuated. Hospitals would be jammed with citizens terrified about radiation poisoning. Emergency quarantine rules might have to be instituted. Government departments would have to move premises in a hurry, causing immense disruption. Both the President and Congress might have to find new temporary homes, or even leave Washington for secret underground locations.

Move the premise to another city, such as New York, and the nature of the havoc changes slightly, but not its extent. A strike on the financial district or midtown might have less symbolic value ­ apart from that of New York being lined up again for attack ­ but just imagine the economic disruption in a commercial powerhouse of a city, densely populated with big corporations.

Or imagine another scenario, raised in The New York Times recently by the Russian nuclear physicist and radiation clean-up expert Vladimir Shikalov: a strike on Disneyland. Again, the immediate human toll might be relatively low, but the panic would be so great, and the lingering low-level radioactive contamination so hard to eliminate, that the Magic Kingdom might well be forced to close for ever. It would, as the New York Times reporter Bill Keller put it, "constitute a staggering strike at Americans' sense of innocence".

There is, of course, nothing new about the idea of a dirty bomb explosion on American soil, or anywhere else for that matter. What has changed since 11 September is the realisation in high government circles that the theory is a lot closer to being put into practice than previously imagined.

Mr Muhajir's arrest appears to provide the first concrete evidence that a dirty bomb plot was in the works. For about three months, however, US government experts have recognised that al-Qa'ida probably has access to both caesium-137 and strontium-90 and has the capability to use them to construct a dirty bomb. Their assessment is based both on written materials recovered after al-Qa'ida and the Taliban fled Kabul and other cities in Afghanistan at the end of last year, and on the alleged testimony of Abu Zubeida, a senior al-Qa'ida operative captured in Pakistan in March.

According to media accounts of his interrogation, Mr Zubeida alluded to a dirty bomb plot right from the start. It took a couple of months for US officials to cross-check his claims and take them seriously enough to tail Mr Muhajir, who was arrested at O'Hare airport in Chicago on 8 May. That the arrest has only now been made public suggests several more weeks were needed to assemble enough further corroborating evidence to be sure the threat was real.

The news of Mr Muhajir's arrest comes at a time of extreme public nervousness in America over what kind of attack might be coming next. Yesterday, The Washington Post divulged details of a government report into air freight transport that revealed a near-absence of security checks on cargo consignments. Keller's recent article in The New York Times magazine, meanwhile, speculated that the prospect of a full-blown nuclear attack on American soil was not so much a matter of if, but when.

The article uncovered yet another lax security area, this time the containers brought on board ship into US ports. Fewer than 2 per cent are opened for inspection, and most never pass through an X-ray machine, Keller reported. Containers delivered to up-river ports such as St Louis or Chicago pass many potential targets before they even reach customs.

Ultimately, the public nervousness has less to do with media stories than it does with the torrent of revelations about the intelligence failures leading up to 11 September. The FBI and CIA ­ once feared, if anything, for excessive use of their powers ­ have been exposed in the past few weeks as feuding rivals of alarmingly limited competence, more interested in waging turf wars than in preventing future atrocities.

As for the Bush administration, there are broadening suspicions that it has more ideas on spin-doctoring the crisis than it does on actually confronting it. Mr Muhajir's arrest is a rare breakthrough in an investigative record that could boast only three other high-profile detentions: John Walker Lindh, the so-called "American Taliban", Zacarias Moussaoui, the suspected "20th hijacker" in the September 11 plot, and Mr Zubeida.

The nervousness is not as palpable as it was, say, at the height of the anthrax panic last autumn, when there was a nationwide run on gas masks, tinned food, bottled water and antibiotics. Part of the nature of the new mood of grim inevitability is the sense that there is little, practically, that anyone can do. Americans' gut instinct at such times is to put their trust in government, and President Bush's continuing high poll ratings attest that they are continuing to do so. But government is also being shown to be falling demonstrably short and, for the first time since 11 September, there is rank cynicism about the effectiveness of the administration's proposed measures, such as the creation of an overarching domestic security agency.

As one columnist, Rob Morse of the San Francisco Chronicle, put it: "None of us should be reassured about anything. As shown by its recent mixed messages about terrorist threats, the White House doesn't know anything. If the FBI knows anything, it's in a memo buried on some bureaucrat's desk."

The picture that has emerged over the past few weeks is of a country almost lackadaisically uninterested in counter-terrorism until it was too late. On 10 September, the Attorney General, John Ashcroft, turned down a request to boost the FBI's counter-terrorism budget by $58m (£40m). According to yesterday's Los Angeles Times, FBI agents at the Phoenix field office were taken off counter-terrorism assignments last summer and diverted to a series of arson attacks.

Until 11 September, counter-terrorism was the number four priority at the Phoenix office, behind mafia activity, white-collar crime and crime on Indian reservations.

A now much-cited memo from a Phoenix field officer on suspicious Middle Eastern men training at flight schools, filed last July, was roundly ignored by FBI higher-ups in Washington.