Three 16-year-old girls strangled, a country's biggest murder hunt and the ground-breaking way police found the serial killer - this is the real life story behind the BBC's latest drama.

Many feared detectives would never find the killer known as the "Saturday Night Strangler" - but 30 years on, he was found in the most unlikely place.

It took two DNA-firsts to find the man who had struck fear into south Wales.

It all started in the summer of 1973, when the city and surrounding areas of Swansea were shocked by two incidents of girls going out partying but not returning home.

In July, 16-year-old Sandra Newton had been out with her boyfriend in nearby Briton Ferry when she disappeared on her five-mile walk home at about one o'clock in the morning.

Police believed she had tried to hitchhike her way home.

Sandra's body was found two days later in a culvert. She had been hit over the head and strangled with her own skirt.

Then, in September, Geraldine Hughes and Pauline Floyd were found dead in woodland in nearby Llandarcy after hitchhiking their way home from a night out in Swansea.

The 16-year-old best friends had been beaten, raped and strangled, and their bodies dumped just seven miles from where Sandra was found.

Their deaths sparked Wales' biggest murder hunt and a team of 150 officers questioned 35,000 people who loosely fitted the description of the last person seen with Geraldine and Pauline.

But having bushy hair, a moustache and being a man between 30 and 35 years of age was not the only lead for investigators - detectives also had a description of the car he had been driving.

Their key witness had seen the girls leaving Swansea that night, getting into a light coloured Morris 1100 being driven by a man.

As a BBC reporter, I helped to cover these killings back in 1973.

They stunned an area where murders rarely happened.

The shock of young girls going out for fun and never getting home created a real sense of fear - and this man was still at large.

There was a tremendous outpouring of grief. The communities were desperate for somebody to be caught - but no-one was arrested and there were no strong leads.

There were so many areas where the killer could have come from.

Working without computers, police faced an impossible task as they sifted through a huge pool of possible suspects from a mountain of paperwork.

Officers, at the time, considered that the same person had killed all three girls - but someone was already in the frame for Sandra's murder.

The prime suspect for Sandra's death was the last person who saw her. That was her boyfriend, but he maintained his innocence and was never charged.

Despite the brutal similarities, detectives continued to run separate inquiries into Sandra's death and the murders of Geraldine and Pauline - until almost 30 years later.

Both investigations were linked in the early 2000s by an emerging crime-solving tool, when South Wales Police started reopening cold cases hoping DNA could finally bring killers to justice.

Firstly, semen stains on Geraldine and Pauline's clothing were isolated to show they matched that of the same man - but the person was not on the recently opened DNA database.

But a year into the new Operation Magnum inquiry, tests on Sandra's underwear gave detectives their first major breakthrough.

There was DNA present from an unknown male.

"I recognised straight away from a particular feature of the DNA - this was the Llandarcy killer," said forensic scientist Dr Colin Dark.

"This was an absolute bombshell because this meant there was a serial killer operating in south Wales in 1973 killing young girls."

The breakthrough also categorically proved Sandra's old boyfriend at the time of her death was completely innocent.

"This was the first time in almost 30 years we knew that the same man had killed all three girls," said the man that led the new investigation, Det Insp Paul Bethell.

Because there was no matching sample on the national database, the killer's identity still remained a mystery and detectives asked BBC Crimewatch for help.

But scientists had come up with another trailblazing way of using DNA to catch the murderer.

"We started to think was it possible that we could use the idea that crime can run in families," said Dr Dark, whose team was continually checking the sample with new profiles on the ever-growing DNA database.

"You inherit your DNA from your parents and you pass your DNA onto a child. So could we look on the DNA database for a child of the offender? This was a definite possibility."

Dr Dark said that meant "getting a spreadsheet printout of several thousand DNA profiles from men in the south Wales area, sitting down with a pencil and a ruler and crossing out everyone that doesn't match."

"After several hours of going through the process, we were left with about 100 names. They were all exact half matches to the offender's profile. So they were potentially children of the offender," he said.

"This was a ground breaking technique, the first time it's ever been done in the UK and possibly the world - and from there, the new investigative tool now known as familial DNA was developed."

Meanwhile, detectives had painstakingly whittled down the list of 35,000 possible killers to just 500 prime suspects.

"Our shortlist was based on the description of the individual, did the owner have a 1100 car and did they have previous convictions for violence or sexual offences," recalled Det Insp Bethell.

After cross referencing both lists, one surname stood out - Kappen.

Local car thief Paul Kappen's DNA was on the database after committing offences in and around the Port Talbot area, but he was only seven at the time of the murders.

His dad, however, had been questioned back in 1973 because he fitted the description of the wanted man - and drove a light-coloured Morris 1100.

That same year, police visited nightclub bouncer and part-time bus driver Joseph (Joe) Kappen's house on Port Talbot's Sandfields council estate.

But he had an alibi from his wife and claimed his car was broken down on the night of the murders.

"He was known in the community as a bit of a thug," recalled Det Ch Insp Bethell.

"He had a history of domestic violence and had been to prison on a number of occasions."

Now, with his son's DNA proving he was a 50% match of the killer's DNA, Joseph Kappen became the prime suspect - decades after the murders.

But there was a problem getting a DNA swab from him when they knocked on his door.

Kappen had died of lung cancer 11 years earlier in 1990, aged 48.

To confirm police suspicions, scientists requested DNA swabs from Kappen's ex-wife and daughter to compare them to the killer's sample.

"That gave us two-thirds of a full profile for Joe Kappen," said Dr Dark, whose team were based at the old forensic laboratory in Chepstow, Monmouthshire.

"When we compared that to the crime stain profile, we could see that matched - but we needed the full profile to make absolutely sure that we had identified the killer."

Detectives were determined to be absolutely certain that Kappen was the killer so the families of the three girls could have some sort of closure.

That meant exhuming Kappen's body, which would be the first time that a suspect would be exhumed in the UK to prove guilt.

Detectives needed to go to the top for permission and seek the home secretary's approval.

"My main concern was whether there was sufficient evidence because it is a big step as if you get it wrong, the families involved would rightly be aggrieved," said Lord Blunkett, home secretary between 2001 and 2004.

"My decision was very clear that Joseph Kappen's body should be exhumed to test once and for all that this was the man who committed the murders and bring whatever comfort we could to the families. For the sake of everyone, finding the truth was really important."

So history was made in May 2002, when exhumation of the three-person grave began on the stroke of midnight.

"It was a horrible night and just as we got to Kappen's coffin, there was a clap of thunder," recalled Dr Dark.

"It was the feeling that evil had been identified, it sent shivers down my spine."

Forensic DNA examination at nearby Morrison hospital in Swansea proved Kappen was the killer of all three girls, after almost three decades.

"No words can describe the way we all felt, it was a huge relief," said Geraldine's cousin, Julie Begley.

"None of us ever stopped hoping that one day we would find out who he was. Although you get on with your life, it never goes away.

"Geraldine was a fantastic girl. She was always full of fun."

It was not just closure for Geraldine and Pauline's loved ones, but for Sandra's family and close friends too.

"I've been going down to her grave for 49 years and it's hard to believe she's there," said Sandra's friend Theresa May.

"My beautiful friend. She had her life taken away from her, her future taken away from her.

"I still think about her now. All these things that we could have and should have done together.

"But after all of those years, she can finally rest in peace now."

Philip Glenister and Steffan Rhodri play the detectives leading the cold case murder investigation, using trailblazing DNA techniques to try to catch the killer after almost 30 years.

The four-part drama and accompanying documentary starts on Monday on BBC One and will be available as a box set on BBC iPlayer.