OTHERS have killed more, but no one has ever scared so many so much in America as the Washington sniper has since October 2.

He -- if the shootings that have killed nine people and injured two others, including a 13-year-old boy, are the work of a lone gunman -- is already the most terrifying serial killer in American history.

At the moment, investigators have few leads. But according to Cary Cooper, professor of psychology at the University of Manchester Institute of Science and Technology (UMIST), the sniper displays all the characteristics of a classic psychopath.

"He is probably psychopathic, in the sense that he has no conscience about what he is doing or who he is killing, whether it is women or men," he says.

"He is also at a fair distance from them, so in a way he is not actually seeing what he is doing."

Prof Cooper says the sniper's skill with a weapon -- using only one bullet in each of the shootings -- suggests he may have a military background, or may have been a member of a gun club or a policeman. But there are very few clues as to why he has embarked on such an apparently random killing spree.

"He is obviously angry at something or somebody, although I don't know who that is. He may have been thrown out of the military, or of a gun club, or he may have been rejected when he tried to join the police."

The killings have brought a state of paralysed fear to Washington DC and the surrounding counties, but with no idea where the sniper will strike next, there is no refuge in knowing which places should be avoided.

Washington residents who coped with the September 11 terrorist attacks and the anthrax scare by going on with daily routines -- putting petrol in the car, taking the kids to school, shopping -- now find those routines targeted by a maniacal sharpshooter.

The shootings have not scared everyone, but they have affected almost everyone directly or indirectly.

They have prompted a number of coping strategies. Fill-up the car inside Washington, where the sniper has struck just once, not in the suburbs.

Some men take their wives' cars to the petrol station for them, and many pull up to the full service island and stay behind the wheel. Couples go to restaurants with valet parking. Children listen to the whirr of helicopters at night and ask, as they're tucked in, about the bad man.

And everyone is looking for a certain white van or a box truck, both of which were seen at shooting scenes.

Schools across the region extended a ban on all outdoor activities

Peapod, a company that takes online grocery delivery orders, says business in the region has risen sharply. Typically, the company says, it can deliver groceries ordered by noon within 24 hours. But demand has been so great that food is not arriving until the next afternoon or evening.

Meanwhile, other businesses are hurting. By 4pm, the Dollar Discount store has usually had about 500 customers. By 4pm on Tuesday, it had 10 -- all of whom talked about you know what.

Everywhere, there are signs of tension. A father scolds his 17-year-old daughter for standing near a window in a drugstore on Wisconsin Avenue, in Washington.

A woman with a young child in tow is ready to leave a pancake house in Chantilly. She tells the child, "OK, now we're going to look for a white van, and if we don't see one, we're going to run to the car." And they do.

For some, the sniper's biggest impact has been traffic. Miles of cars have sat for hours in jams created by police blockades after a shooting.

For Prof Cooper, the panicked response to the sniper's random attacks may fuel the desires which brought him to launch his killing spree.

"It makes him feel powerful. He now has the whole area frightened of him, and that is giving him a sense of power and influence, that maybe he doesn't have in his ordinary life.

"People are terrified. To a distorted mind, this means 'Look how powerful I am. I have the capital of the United States frightened to death of me'."

Chillingly, Prof Cooper suggests this means the sniper will not stop until he is caught.

"He will carry on until he is stopped. He is not going to stop. If the objective is to show the power and influence he has, and that he can have a whole community living in fear, then there is no reason to stop.

"The only reason he might stop is if he moved away, and then he'll do it somewhere else."

But Prof Cooper says there may be a glimmer of hope for those detectives who have so far found themselves several steps behind the sniper.

"He will slip up, and they will find him. It may be because he wants people to know and that could be his downfall."