The young man left the top floor council flat and glanced nervously around him before heading out into the darkness of the night.

It wasn't just the winter cold that was causing 22-year-old Zhi Min Chen to shiver.

Minutes earlier the illegal immigrant from China had murdered 21-year-old sex worker Tracey Wylde after meeting her in Glasgow's notorious city centre red light district.

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Unusually the young mother had taken this customer back to her home in Torryburn Road, Barmulloch.

It was a decision that would prove fatal.

Their brief encounter turned violent with cowardly Chen strangling Tracey, leaving her three-year-old daughter Megan without a mother.

There were no witnesses that November night in Glasgow in 1997 to the brutal attack. No one saw Chen arrive at or leave Tracey's home.

Neighbours had heard arguing from the flat in the early hours of November 24, then the sound of a door slamming and keys being rattled.

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Detectives discovered that on the previous night, November 23, Tracey had gone into Glasgow city centre.

She was last seen on CCTV in Wellington Street around 3.20am with a mystery man who appeared to have an arm around her shoulder.

An hour later a neighbour heard arguing coming from her flat and Tracey shouting for the woman's mother.

The neighbour went to investigate but she could not see into Tracey's home and didn't hear any further shouting.

The alarm was raised later that day when the young mother, who was battling drug addiction, failed to attend a support group meeting.

One of the support workers went to her flat and noticed a balcony door was open.

She alerted the same neighbour, who managed to get into the flat, and they both found Tracey dead in her bedroom.

She was lying on her back with a dressing gown over her body, fully dressed with marks and bruising around her neck.

The murder inquiry was one of the biggest of its type and lasted two-and-a-half years with DNA being taken from around 2500 men.

A few days after her death Tracey's mother Fay McCash made a moving appeal for information on her daughter's killer. She told reporters that she hadn't known her daughter was a sex worker.

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In January detectives released the CCTV showing Tracey with the mystery man in Wellington Street.

In a new move for the time police also began taking DNA swabs from men caught using prostitutes or known to use their services which were then compared to samples found at the murder scene.

Tracey was one of six sex workers killed in Glasgow in as many years But police were certain that none of the fatal attacks were linked.

In most cases, men had been charged or stood trial.

But a lack of evidence meant no convictions in any of the six cases at that stage.

However, they were keen to bring Tracey's murderer to justice and dispel rumours of a Ripper-style serial killer.

However, despite the best efforts of Strathclyde Police they met a wall of silence in their bid to find Tracey's killer.

None of the DNA or fingerprint evidence from the crime scene matched anything held on file.

The women who worked the streets were mainly drug addicts like Tracey and were unable to give much useful information.

In March 1998 BBC's Crimewatch featured Tracey's death.

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In what would later prove significant one woman phoned the studio to say she believed the man seen in the CCTV footage lived in student accommodation in the city.

However, Chen, who had come to Scotland in 1995, remained free.

Being an illegal immigrant he did not technically exist and no one was likely to admit knowing or employing him.

As the years passed Chen married, set up his own takeaway business, and became a respectable member of the community.

With two children and a home in Anniesland he would have every reason to believe his dark secret would continue to remain undiscovered.

A cold case review was carried out in 2013, by the newly formed Police Scotland, which looked at the original CCTV and DNA evidence from the case.

By now it had become one of Scotland's most notorious unsolved murders.

Using the latest scientific and forensic techniques police then found traces of "foreign" DNA on Tracey's body.

Up until then, the assumption had been that the killer was a local man.

As a result of the new information, police gathered details of all overseas students who were in Glasgow around the time of the murder.

In a bizarre twist detectives finally uncovered a suspect.

However it wasn't Chen but an Indian man who had been a teenage student in Glasgow at the time of Tracey's murder.

In January 2015 the suspect was detained at his home in Mumbai after an extradition request from Scotland He was given bail three weeks later on the condition he surrenders his passport and reports to the police once a week.

The suspect was by now married with three children.

He'd arrived in Scotland in August 1997 to pursue a one-year degree course at Glasgow College of Nautical Studies and lived in student accommodation.

Around eight months later he gave up the course and flew back to India to do a degree course there..

Detectives believed he was the man seen with Tracey in the Wellington Street CCTV footage.

Chen - if he was aware of the latest development - would have felt even more secure as the Crown Office pursued the Indian man's extradition.

However, his secret life was to suddenly unravel thanks to the efforts of the original murder forensic team.

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Purely by chance Chen had been arrested and charged with two assaults in Cowcaddens in July 2018. When his fingerprints were routinely put into the database they immediately flagged up a match with prints and DNA found in Tracey's flat.

Chen was then charged with the murder and appeared at Glasgow Sheriff Court where he was remanded in custody.

The following April, Chen, now 44, admitted the murder at the High Court in Glasgow.

A few days later the Crown Office confirmed they were no longer pursuing the extradition of the Indian student.

Tracey's daughter Megan Smith, now 25, was in court to see Chen admit his guilt.

In an interview in May 2019, she said he should have handed himself into police immediately after the murder.

Megan added: "Chen would've saved my family a lot of pain over the last 22 years.

"I've finally got justice for my mum and feel I can get on with the rest of my life now.

"I was never bothered about what my mum did. She was a human being and my mother and it didn't make her different from anyone else."

At the end of the case police were able return some of Tracey's possessions to Megan, particularly a photograph of the victim and daughter — the only photograph that existed of them both together.

On May 18, 2019, at the High Court in Glasgow Chen was sentenced to life imprisonment.

His lawyer Donald Findlay was unable to offer a reason or motive for the murder other than the couple had a row.

Chen kept his head bowed throughout the proceedings and did not look at Tracey's relatives as he was taken down Tragically Tacey Wylde's mother Fay McCash never lived to see her daughter's killer brought to justice having died from cancer in February that year.

A few days after Chen's conviction the Mumbai man arrested for the murder said it had ruined his life.

The former student was unable to get a job while the extradition hung over him and had received no apology from the police.

In one interview at the time he added: "I have never used a prostitute.

"I never met Tracey. The first time I ever saw her was when I was shown a picture in jail. I was arrested for no reason."

Neither the police nor the Crown Office explained why they had gone to such efforts to try and extradite the Indian suspect.

In a final twist to the case Chen successfully appealed the minimum period he must serve in prison before he is eligible for parole.

At a hearing in Edinburgh in October, 2019, three appeal judges reduced it from 20 years to 16.

However, Lord Menzies warned Chen that wouldn't mean he'd be automatically released after serving the lesser period.

He added ominously: "It's possible that a person serving a life sentence is never released from prison."