EVEN the prosecution was at a loss to explain what had driven Barry George to murder the pretty, blonde television presenter with whom he was obsessed.

They had all the evidence, all the facts, all the forensic science, but the biggest gap in their case was the answer to the small word 'why'.

As prosecutor Orlando Pownall admitted, the motive for George's actions was "impossible to determine with any degree of certainty".

Yet for celebrities around the world, attack by an obsessed stalker is a major fear - and it must be a major fear because during the course of their investigation into Jill Dando's death, police discovered another 140 people who had an unhealthy interest in her.

Indeed, George was not the first stalker to have caused Miss Dando real problems. Three years before her death, bachelor John Hole admitted sending her Valentine cards, waiting for her outside the BBC studios, phoning her at work and even calling at her Fulham home. His behaviour concerned the BBC so much that it contacted him, saying his actions constituted harassment.

A year before her death, though, Miss Dando was again subjected to frightening phone calls and letters. This new stalker put a note through her front door after she was pictured with fiance Alan Farthing.

At the tenth anniversary party of her Crimewatch programme in 1999, Miss Dando spoke of her worry about her safety and Alan Yentob, the BBC's director of television, was told of her concerns.

But her co-presenter Nick Ross said she regarded the stalker "only in the sense of an irritant". He said: "She never said to me that she was frightened or fearful, that something terrible could come of this."

Yet it concerned her enough to take a great interest in her home security and personal safety. In a prophetic interview for the BBC's website shortly before her death, she said: "It upsets me that there are such brutal people. Occasionally we get the stories behind the crimes and the motives are quite sickening. I take great care over home security and about walking home in the dark."

And all of this misplaced obsession was with Miss Dando who, although a warmly-regarded early evening star on British television, did not receive the global attention of a top rock star like Madonna or a top sports star like Monica Seles.

The GMTV presenter Fiona Phillips said that Jill Dando's death had brought home to her how vulnerable she was, too. On the day Miss Dando died, she went to put the rubbish out and suddenly thought: ''My God, someone could shoot me now. It could easily happen to me.''

It did happen to John Lennon. On December 8, 1980, the former Beatle was returning to his apartment in New York City, when he was gunned down by obsessed fan Mark Chapman. Chapman believed his actions would help him achieve immortality.

Twenty years later another Beatle, George Harrison, was repeatedly stabbed at his home by a disturbed fan who thought the rock star was a witch after listening over and over again to the lyrics of the group's songs.

Young singer Billie Piper, now the wife of troubled DJ Chris Evans, was forced to take legal action against a fan after death threats were left on her record company's answering machine.

Teenage star Britney Spears was the victim of a deranged fan who sent pornographic messages to her Internet site. A man also photographed himself in the bath of the home the American performer she was having built.

Actor Brad Pitt has also been a target - he was horrified to discover a young woman stalker had broken into his home, worn his clothes and slept in his bed.

The late Diana, Princess of Wales, found herself the victim of a number of stalkers, most notably Dr Klaus Wagner - and although it is not believed that Barry George stalked Diana, he certainly had an unhealthy interest in her.

Meanwhile, Monica Seles, the one-time world number one women's tennis player, was stabbed at a tournament in Hamburg in 1993. The perpetrator was Gunther Parche, a fan fixated with his victim's main rival, Steffi Graf. Others who have suffered include Victoria Beckham and footballer Ryan Giggs.

But, more worryingly, experts believe that it is not just the well-known who are at risk from the bizarre obsession of others. Experts believe that about 20 per cent of women and two per cent of men will be stalked at some point in their lives.

The estate agent Suzy Lamplugh is believed to have been stalked before she was kidnapped. The man, known as Mr Kipper, had phoned her and sent flowers.

Research carried out by Leicester University suggests that violence is a common outcome in stalking cases. Of the victims researchers had spoken to, 22 per cent had been subjected to a murder attempt, 36 per cent had been attacked physically and 58 per cent had been threatened.

Stalking is, though, an activity which only became a crime in 1997 with the passing of the Protection from Harassment Act. In 1999, there were more than 5,000 convictions under the Act, and experts believe that had it existed earlier a lot of unsolved killings may have been classified as stalker murders.

Metropolitan Police Detective Inspector Hamish Brown, the nation's foremost police expert on stalking, said there is no blueprint for stalkers. What they tended to have in common was a belief that in some way, they had a relationship with the victim.

Evonne Von Heeussen, director of National Association for the Support of Victims and Survivors of Stalking and Harassment (NASH), says stalkers fall into three distinct areas.

The first, comprising about 85 per cent of cases, is known as post-relationship stalking - an ex-boyfriend or partner who refuses to believe the relationship is over.

The second category, accounting for about ten per cent of cases, is the casual contact stalker. "They will first see you at an evening class, or at a party or through work. Then you start going to the pub and they're there, or going swimming and they're there. They will then try to establish some rapport and, if you reject them, could get quite nasty."

Those which fall into the third "stranger contact" category are the type who stalk celebrities. They are the obsessive fans who believe their objects of desire are in love with them but they're just playing hard to get. George would fit either of the last two profiles.

Evonne was stalked for 17 years by a university lecturer and, like Miss Dando, at first, hadn't a clue. "I didn't know for the first three years what was happening. I was getting packages of photographs," she recalls.

But just as no one knows what tips someone from a having an interest in another person into stalking them, so no one is sure what tips a stalker like Barry George into taking violent action against them.

Chartered psychologist at Newcastle University Dr Joan Harvey says: "Lots of people are obsessive. George is beyond that. Clearly, he's way out of the realms of normality.

"There is big debate about what is in people to start with, how far people can be socialised into good and bad behaviour. But the jury is less out than it was. There is no doubt that you are not simply someone who is moulded by society. With people like this, there is no doubt the potential is in there to start with."

How that potential develops is unknown. As Justice Gage said yesterday as he sentenced George: ''You are unpredictable and dangerous. Why you did it will never be known."