Personal alarms, the little devices which give out a high-pitched screech designed to fend off attackers, sold out in one shop in Dun Laoghaire this week. Worried parents and women who live alone or have to go out unaccompanied were buying the devices, which fit easily into handbags. Locksmiths and security alarm companies were also experiencing brisk business.

Girls who normally walk home from school in this part of south Dublin are being collected by parents in cars.

The evening streets and roads are empty of strolling lone teenagers. Their absence is noticeable. So, too, are the nervous glances at strange men in quiet streets that have replaced the normal nonchalance.

The relaxed atmosphere of the summer evenings disappeared with the murder of Raonaid Murray, the 17-year-old girl with long blond hair who was stabbed to death as she walked home from the centre of Dun Laoghaire a week ago last night.

Parents of independent-minded girls like Raonaid say their daughters have become introverted and sleepless from fear.

Raonaid had an extraordinarily large circle of friends and acquaintances. At her funeral were girls and boys who knew her from her schooldays and young people who had known her from the pubs and discos in the town centre. Hundreds of them came to her funeral.

Many had been in Scotts pub in George's Street last Friday night when she had come over from her work in one of the boutiques in the shopping centre. She spent 2 1/2 hours over a few drinks with a few close friends.

Scotts is one of the most popular after-work meeting places for young salespeople from the town's shops. Raonaid left at around 11.30 p.m., parting from one of her friends at the door. And from that point there is still mystery.

All that is known is that she made her way as far as a laneway that runs from Silchester Road to Silchester Park, where she lived with her parents, brother and sister. Her friends told gardai that she was accustomed to taking one or other of the routes home.

On her own, she might have walked to the end of George's Street and turned right at the People's Park up Lower Glenageary Road. Another route would take her up Northumberland Avenue, through a narrow lane at the top on to Corrig Avenue and then across Crosthwaite Park.

She also often walked from Glasthule along the Metals, the path running alongside the DART line, and then through another laneway to Silchester Road. The Garda search team stripped the ground along both the laneway where she was killed and this other lane leading down to the Metals.

The route to her home from Scotts through Crosthwaite Park takes 15 minutes. The other is less direct and takes longer. Either route would take her through deserted streets with dark tree-lined stretches. She could easily have been followed along either route.

Raonaid was wearing platform shoes, and with the noise from these and the rustling of the paper fashion bag she was carrying, she might not have noticed a stalker behind.

The weather around south Dublin last Friday night was sultry and still. A stalker would have to be particularly light of foot not to have been noticed in the quiet streets between the town centre and Glenageary.

It is also possible her murderer was waiting, lurking behind one of the sycamores in the laneway, and stepped out as she walked into the shadows of the lane.

The detectives investigating her murder, based at Dun Laoghaire Garda station, have had great difficulty piecing together her last movements.

They were not even sure that she had walked home. Every taxi-driver who was working in the Dun Laoghaire area on Friday night was checked to see if any had had Raonaid as a fare.

The times of the known events have also presented detectives with further problems in understanding what happened. If she left Scotts at 11.30 p.m. and went directly home she should have been walking into the lane in Silchester at 11.45 p.m. But there is a sighting of her in Lower Glenageary Road at about 12.05 a.m.

The lane runs between houses and past the courts of Glenageary Tennis Club, and the gardai have a reliable report from one of the residents of hearing a scream at 12.10 a.m. The time lapse between her leaving the pub and arriving in Lower Glenageary Road has perplexed detectives throughout the week.

And there is the problem over the killer's motivation. There was no attempt at robbery or sexual assault. And Raonaid appeared not to have had an enemy in the world. She was cherished by her family and friends.

One of her group described her as the most liked person of their social circle. She was ebullient and chatty, and was as popular among school friends as she was among the disco and pub crowd in Dun Laoghaire.

What was clear was that the person who stabbed her went out that night with the intention of committing murder. The knife was long and wide, possibly an inch-and-a-half wide.

Any wound from such a weapon is potentially life-threatening. Any of the four blows which struck Raonaid could have killed her.

But despite the savagery of the attack she was still able to stagger 100 yards out of the laneway and into Silchester Gardens before falling and dying on the pavement.

The evil, premeditated nature of the murder struck terror into the hearts of young people in the area. The attack bears a horrible resemblance to scenes in popular horror movies such as Scream, I Know What You Did Last Summer, Halloween, and its sequel, H20.

Scenes from these movies would be vivid in the minds of the teenagers who watch them. Even the poorly lit, misty night-time scene in the lane off Silchester Road is straight out of a horror movie. Raonaid's murder must have evoked dreadful images in their minds.

The brutal nature of the murder, and the fear that the killer might strike again, led the gardai to launch one of the biggest murder investigations since the death of Veronica Guerin.

Some 50 detectives and forensic and technical specialists have been involved since the investigation started. The Garda Commissioner, Mr Pat Byrne, directed that whatever resources were necessary would be provided.

By yesterday morning detectives had taken 600 statements and carried out house-to-house inquiries from Dun Laoghaire to Glenageary and in Blackrock, where a man who took a taxi from Dun Laoghaire was dropped in the early hours of Saturday morning.

Last night the Garda swamped Dun Laoghaire and its environs, stopping cars and pedestrians. They visited pubs and clubs which Raonaid and her friends frequented, showing a photofit of the man who took the taxi from Dun Laoghaire to Blackrock.

By last night there was still no clear sign of the investigation coming to a quick end.

"There's very little evidence. We're hoping for a break," one of the officers said.