Their obsession with their female victims is chilling. They can leave lives in ruins. In Los Angeles is the world's only police team dedicated to pursuing them. James Cutler and Rachel Coughlan see the stalker squad in action
The Government's proposal, announced last week, to make stalking a crime, followed months of media pressure for something to be done about this terrifying behaviour. An earlier attempt by Janet Anderson to introduce a Private Member's Bill was deemed by the Government to be unworkable and thrown out. But how easy is it to legislate for this crime, especially when evidence suggests different grades of stalker?

When we started to film with the Los Angeles Police Department's threat management unit - the world's only police team devoted to pursuing stalkers - for our Inside Story documentary, "Stalking the Stalkers" (BBC1, tomorrow, 9.30pm),we expected many of their cases to feature obsessed fans tracking down Tinseltown celebrities, or deranged individuals latching on to complete strangers after a chance encounter.

There were cases that fitted into these categories. The unit - known, inevitably, as the stalker squad - handles some of the biggest names in show business. It is rumoured that you know you've made it in Hollywood when you've got a stalker. The only people who tell that one are the ones who haven't made it. Big stars know only too well how far a fanatical fan can go. In fact, the bigger a star you are, the more unwilling you will be to talk about this problem. Stars, and more importantly their agents, feel that talking publicly about stalking will damage a carefully nurtured public image. That is why it's a secret that as soon as Madonna's stalker, Robert Hoskins, was jailed for 10 years in March, another appeared, his attentions directed at her record company and staff, while she was out of town filming Evita.

Stars have to be careful, and they can pay a high price, literally, for their safety. We heard the story of one of Hollywood's hottest actresses who paid out $2,000 a time to a private security company to be escorted from the airport after noticing that she was being tailed by a mysterious young man. Advised tht the services of the stalker squad were free, she asked them to investigage. They found out that the young man was a spotter for paparazzi. He was warned off, but not prosecuted. The LAPD has no difficulty distinguishing genuine stalkers from journalists, one of the problems identified in the Home Office's consultation paper.

Another well known Hollywood-based singing star probably keeps her stalking situation quiet out of embarrassment at the way in which it arose. A feature of her concerts was an intimate little chat with the audience, bemoaning the fact that she was lonely and inviting men to send her their proposals for solving her problem. Now she's a client of the threat management unit, which tracks her stalker from state to state as he follows her concert tours.

However, most of the cases handled by the unit are far more complex because they arise out of prior relationships and it is here that our legislators can learn many lessons. Like murder and rape, stalking is rarely a "stranger crime". In most cases it is men who stalk women, men who cannot tolerate rejection and loss of control. The fact that the victim almost always knows her tormentor doesn't make it any less terrifying for her. In fact, he probably knows exactly which buttons to press to terrify her more. It also means that the police have to have a high level of understanding of the complexities of human relationships and to understand the dynamics involved when it comes down to arrest and prosecution. To help them with this, the detectives receive on-going training and have their own forensic psychiatrist attached to the unit.

They believe that, at any time, anyone who is unstable enough to be obsessed with someone else has the potential to become violent. You won't hear them commenting, as a British sergeant did recently, about a man stalking a 13-year-old girl, "I don't think he's dangerous." Even in LA, where the laws have been in force for six years, the threat management unit finds it has a lot of educational work to do with ordinary cops on the beat, who are likely to be the first port of call for any victim.

When Marla Frees, a young actress (featured in the BBC documentary) from shows such as Cybil and Roseanne, contacted her local police station after her stalker had violated the restraining order she had taken out against him, she was horrified by the response. "They were clueless and told me they couldn't enforce it. I said to them, 'What do I have to do, come back here in a bodybag?'" The stalker squad works hard to counter such attitudes.

Another lesson learned by the unit is that the one thing that defines stalking behaviour is persistence. Lieutenant John Lane, who heads the squad, says: "The police and courts can't necessarily make this problem go away. When the guys are released from jail, they're going to go right back after their victim."

Bob Lee has been stalking secretary Lynne Holdsworth for nearly 20 years. After violating a restraining order, he is picked up by the police. "I'll do my time in jail," he says in the documentary as he is handcuffed to a bench, "but she'll do her time in hell." Bob, a former software engineer, is too clever to make overt threats. But he leaves creepy messages on her answering machine telling her he won't hurt her. "If I wanted to hurt you, I would have done it a long time ago," he says. Sentenced to six weeks, he was out in three and moved straight back into his apartment overlooking Lynne's house where he spies on her. Bob is careful to stay just outside the 50 yards specified in the restraining order. Yet, given Bob's history of psychiatric disorder and an alcohol problem, things could get out of hand.

That is why Lieutenant Lane and his team are looking at more hi-tech solutions. Already in use in neighbouring Orange County is the Stalker Shield, a device which is installed in the victim's home and which is activated by an electronic tag on the ankle of the convicted stalker should he approach while on probation. Planned for the future is a surgically implanted microchip that can be tracked by satellite, which will have the capacity to track up to a million offenders. It is even envisaged that if the stalker is foolish enough to violate his parole, he could be disabled by remotely activating a drug within the chip.

Such solutions may seem drastic - and even downright dangerous, especially if the stalker is driving at the time, but we saw the stalker squad deal with some very frightening cases. Ollie Walters's case is typical of those on the threat management unit's books, and points up the difficulties of policing this type of crime. She is an intelligent and articulate music teacher who fell in love with a man who was to abuse her throughout her relationship and beyond. She became caught in the cycle of domestic violence, leaving him several times, only to return and be punished for leaving.

She had opportunities to prosecute but couldn't bring herself to do that. When she left for good, she moved several times but he always tracked her down. Because of their past relationship and because Ollie is fundamentally a decent, compassionate person, she agreed to meet him, to talk with him and to try to rationalise things. She says: "Unfortunately, my compassion worked against me and I started to listen to what were lies." Once she let her guard drop, as we see in the film, her kindness was repaid with a vicious assault and kidnap.

Even after her stalker was arrested and charged, she still wanted to forgive him. "If you have any kind of feelings or compassion for anyone, you can't want to put someone behind bars." As Ollie weeps for the man who has caused her so much fear and distress, it is a telling moment. Like many stalker victims who have had relationships with their tormentors, she finds that it is one thing to want the man out of her life, another to see him locked up in jail.

The stalker squad detectives are trained to counsel and support their clients through the whole court process, whether it is obtaining a civil restraining order or giving evidence in criminal proceedings. They are given the time and resources to see a case through from beginning to end. If Michael Howard's legislation is to succeed in practice, our police will have to be equally resourced, trained and motivated.

However, even if some cases fail - as they are bound to do in such a complex area - the evidence from LA is overwhelmingly positive. Lieutenant Lane believes his unit has saved many lives and delivers a warning about the future. "Stalking is not some flash-in-the-pan crime of the Nineties. It is going to be with us for a long time."