By a canal in Amsterdam's red-light district, one of the "window women" is actually a hologram which Dutch police hope will help solve the gruesome murder of a sex worker in 2009.

Peering through a frame out onto passers-by, a human-sized likeness of Bernadett Szabo -- in skimpy shorts and a leopard-print bra, a dragon tattoo covering much of her torso -- taps the pane and fogs the glass with her breath.

The word "HELP" appears, chillingly, on the screen.

"Fifteen years ago, Betty was killed in a horrible way and the investigation was never closed," Amsterdam police spokesman Olav Brink told AFP.

Aged only 19, Hungarian-born Betty was stabbed multiple times in her brothel room in the centuries-old red-light district, known as De Wallen, just months after giving birth to a baby boy.

Despite a large-scale police probe, the case went cold.

During a review, however, police found "promising clues" and decided to reopen the investigation, Brink said.

They hope the likeness of Betty, created with 3D visualisation technology, will jolt the memories of people who may have information about her murder.

'Shocking'

"There are still people who know what has happened to Betty," said Brink, hoping that 15 years on, "people feel freer to share information with the police".

The initiative also aims to raise awareness about the violence faced by sex workers.

Around 78 percent of prostitutes in the Netherlands have faced sexual violence and 60 percent report being physically attacked, according to a 2018 report by Dutch charities and sex worker rights groups.

Concerns about violence also rose during the Covid-19 pandemic, when loss of income forced some prostitutes to continue working illegally and reduced their ability to report crimes to the police.

In the week since the hologram and accompanying information about Betty went on display, the police have seen "that a lot of people are talking about it".

"We find it quite special that Betty can bring attention to her case this way," said Brink.

In the streets of De Wallen, lined with women watching from red-lit window booths, groups of locals and visitors pause and strike up conversations about the unusual display, entitled "Who was Betty?".

Theo, 80, who lives outside Amsterdam, said he read about the project in the papers and "came especially to see it" when he was in the city.

Soyoon Jun, 34, lives near the red-light district, "so it was more shocking for me that there were neighbours who are going through this type of horrendous event".

For Jun, who works at a Christian charity, the hologram made the murder "real".

"It wasn't just information that was given out," Jun explained. "People could feel the helplessness that Betty would have felt."

The police have already "received several tips because of the campaign", Brink confirmed to AFP.

They are still waiting, though, for the "golden tip" that will lead them to the murderer, which comes with a 30,000-euro ($31,600) reward.

Relocation plans

According to Brink, the hologram is a "special way of getting attention for this case" -- including by putting it in De Wallen, which is "one of the busiest places in Amsterdam and probably the whole of the Netherlands".

This may not last, though, since Amsterdam's sex workers may soon lose the centrality and visibility of their windows.

The local government plans to relocate the red-light district to a purpose-built centre south of the city in the hope of reducing petty crime and tourist footfall in De Wallen.

The move is opposed by tens of thousands of locals and sex workers, who are calling instead for better crowd control and surveillance in the existing red-light area.

Miranda K, a 57-year-old who lives near Amsterdam and declined to give her full surname, said the relocation plan was a "pity" because she felt "safe" in De Wallen.

She said the out-of-city centre would be in a "dark" area, whereas De Wallen has "tourists and people and locals and everything here. So I think it's safer".

"For me, it's not just about finding Betty or who was Betty," she said, "but it's about... these other ladies on the streets too."