With few hard clues about the series of killings in Washington's northern suburbs, police have been getting help from experts to try to build up a profile of the sniper.

Psychologists have developed various techniques to analyse patterns of criminal behaviour and make educated guesses about the age, background and personality of those responsible.

[The sniper] He is enjoys the notoriety he has achieved and his ability to outsmart the police. James Fox, US criminologist

"Geographic profilers" are also attempting to track down the sniper by entering the locations of the various shootings into a computer.

According to James Fox, a Boston-based criminologist, the tarot cards reportedly left at one scene - which read "Dear policeman, I am God", according to local media - could reveal something about the killer's personality.

"He may not be God but he is certainly playing that role," Mr Fox told the BBC's World Today programme.

"He enjoys the notoriety he has achieved and his ability to outsmart the police."

Mr Fox says the sniper is likely to be "pretty ordinary, someone who does not get the respect that he thinks he deserves, and through this killing spree he is trying to be in command."

The sniper is probably in his 20s or 30s, Mr Fox conjectures.

Crime locations

Some policemen have also suggested that he is a skilled marksman, since victims have been shot with single bullets.

A recent shooting occurred outside a school

Another precious set of clues for profilers involves the geographical proximity of the attacks.

The first five victims were killed within a few miles of each other in Washington's Maryland suburbs.

The sixth person who died was shot on a street corner in Washington, close to the Maryland state border.

The emerging science of geographic profiling works on the theory that criminals tend to hunt victims in areas they are familiar with.

However criminals tend to avoid areas too close to their homes, where they may be recognised.

Geo-profilers use computers and mapping software to triangulate the likely home of the killer by analysing the sites of the attacks.

With each murder, more data is entered.

"The more killings you have, the better it works," said Andreas Olligschlaeger, president of TruNorth Data Systems, which makes crime analysis software.

"It's an unfortunate fact," he adds. "More people have to die to get a better chance of capturing the killer."

Cross-section

The technique was pioneered a decade ago by Vancouver detective Kim Rossmo.

Maryland police are getting help from the FBI

While doing doctoral research, Mr Rossmo developed a mathematical algorithm that was used as the basis for software now sold by the Vancouver company Environmental Criminology Research Inc.

Typically, if police believe a killer lives in a 25-square-kilometre area (10 sq miles) area, experts believe they can narrow this down to a few streets.

However, investigators have not been able to detect a pattern from the victims themselves - who appear to have been chosen at random.

Of the eight people positively identified as targets so far, there were five men and three women.

In terms of race, the victims are a fairly representative cross-section of the area: four whites, three blacks, one Hispanic, and one native of India.

They ranged in age from 13 to 72.

The sniper clearly was out to kill anyone - rather than members of a particular group.