Driving up Massachusetts Avenue last Tuesday morning, I was struck by the lack of traffic commuting in to Washington from the opposite direction. Then I realised why: people were staying home because they are in fear and dread from the latest outbreak of terrorism here. A year ago, the spread of anthrax in the mail - and the sorting office that deals with my mail was among those infected with spores - created more fear and anxiety in the Washington metropolitan area than the actual 11 September atrocities themselves. This time it is a sniper, who by Monday morning had randomly shot eight people in five days, killing six and critically wounding two.

I have often wondered why Americans seem to panic in these kinds of situations more than, say, Europeans. Is it simply because history has given Europeans more fatalism and a truer sense of their own vulnerability? Or is it the converse, that a young nation shielded by two giant oceans never felt vulnerable until 11 September last year?

You would think that, in a country that has 25,000 gunshot deaths a year, Americans would become inured to the possibility of sudden, violent death. But, as I write, Starbucks has removed all its outside seating. People are not leaving their offices for lunch. "A terrorised community" is how the news channels are describing the area in which I live and work.

But there are even people theorising that the sniper may be a trained terrorist from that terrifying place, Abroad: al-Qaeda, perhaps, spreading new terror in the nation's capital. The Secret Service has been called in, along with the FBI and the Bureau of Alcohol, Tobacco and Firearms; police cars pointedly park outside schools, and John Ashcroft, the attorney general, salivates publicly over the prospect of invoking the federal death penalty. (Because the murderer has crossed state lines, from Maryland to DC itself to Virginia and then back to Maryland, he has broken federal as well as state laws.)

The case has all the FBI and law-enforcement specialists baffled. The gunman appears to be a good shot, if not an expert marksman, and seems to pick his prey and then plan his getaways meticulously. He (or, possibly, she - or even they) first struck at 5.20pm on Wednesday 2 October, subsequently killing five people within a two and a half-mile radius and in a 17-hour time-span; following the first killing, three more were shot dead at three different locations within 56 minutes.

Although in two cases the bullets were too shattered to be ballistically tested, the sniper appears to use .223-calibre bullets - one for each victim - which shatter inside the victims' head or bodies. In the case of the 13-year-old boy who was shot as he arrived at school at 8.09 last Monday morning, the bullet was fired from woodlands 140 yards away; it made a small entry wound in the boy's upper abdomen, and fragments then entered his spleen, liver, pancreas, stomach, lungs and diaphragm. Had he not been helicoptered immediately to hospital he almost certainly would have died.

The likelihood is that the sniper is using a simple .22 rifle - .223-calibre bullets are commonly used in target shooting and in competitions, including the Olympics, and are, as ever, easily available in the US.

The boy was the youngest victim of the shooting spree; the oldest was 72. For what are apparently random attacks, the eight victims represent a remarkable demographic microcosm of the US: three have been women, four men and one a child. The 13-year-old and 72-year-old were both black, while four men shot dead were all white; one of the women was Latina. The eighth victim was a bearded Indian - the 54-year-old Premkumar Walekar, cut down as he filled his taxi's tank with petrol. Schools throughout the area went immediately into "lockdowns" - the doors locked, nobody allowed in or out until identified parents picked up their children. News and panic spread like wildfire. President Bush paused between zipping around the country to Republican fundraisers to condemn "the cowardly and senseless acts of violence". Parents panicked, and that spread to bewildered children unsure of everything except that the world is a dangerous place for them.

So why is that panic so contagious? I have a theory, which brings us back to those wallpaper news channels. The horrors of Vietnam first got through to the American public via television, and these days the kind of saturation coverage of often minor news events that is the norm creates a sense of threat; that sense of urgent unease is what hooks viewers and boosts the ratings. Singular experiences, such as random shootings, become universal and omnipresent: we are all surrounded by snipers, all become the brother/parent/next-door neighbour of that 13-year-old. And we stay home from work, terrified of when Mr Demon will next strike.