It was a manhunt that struck fear into South Wales in the early 1970s and inspired a true crime drama series decades later. The police investigation into Joe Kappen, the man who became known as the “Saturday Night Strangler” after the brutal 1973 slayings of three teenage girls, was one of the largest in the country’s history and it redefined the way forensic science uses DNA to solve cold murder cases around the world. And though the serial killer case in the city of Swansea was ultimately solved three decades later, Kappen never faced justice while he was alive, and his sexual assaults and murders haunted the local community for years. More than four decades after the killings, PEOPLE is looking back at the haunting serial killer case and how it was solved with a revolutionary way of using DNA. The Murders In September 1973, Geraldine Hughes and Pauline Floyd, two 16-year-old best friends, were found dead less than 50 yards apart from each other in a wooded area the morning after they went dancing at the popular Top Rank nightclub in Swansea, according to The Guardian. Autopsies showed the girls had been raped before being strangled and left for dead. According to the newspaper, more than 150 detectives worked the case, representing the largest murder investigation in Welsh history. About a year later, police linked the girls’ murders to the killing of Sandra Newton in July 1973, leading to his “Saturday Night Strangler” moniker due to the nights each murder took place. Detectives found that Kappen, who was eventually identified as the killer decades later, had preyed on the girls outside of nightclubs on Saturday nights and lured them to his car before raping and killing them, according to Yorkshire Live. But though police identified Kappen’s method and had several descriptors of him — his bushy hair, his mustache, his approximate age and the model of his car — the clues proved too broad and, according to the BBC, police were left sifting through a pool of about 35,000 possible suspects. A New Way of Thinking It wasn’t until the early 2000s that detectives were able to narrow down the list of tens of thousands of suspects they had started with. Forensic science capabilities had advanced significantly in the intervening decades, and South Wales police detectives reopened the case because they could now test semen stains found on each of the victims roughly 30 years earlier, according to the BBC. The outlet reported that detectives began exploring “the idea that crime can run in families," according to forensic scientist Dr. Colin Dark, who spoke with the BBC for its Steeltown Murders documentary. Dr. Dark explains in the documentary that forensic scientists narrowed the list of possible suspects by comparing DNA found on the victims with those in the South Wales database, discovering a list of potential matches for the serial killer’s possible children. "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," Dr. Dark explained to the BBC. "This was a groundbreaking technique ... and from there, the new investigative tool now known as familial DNA was developed." Want to keep up with the latest crime coverage? Sign up for PEOPLE's free True Crime newsletter for breaking crime news, ongoing trial coverage and details of intriguing unsolved cases. Subsequently, detectives soon zeroed in on Kappen, who was originally questioned by police in 1973 because his physical description and vehicle matched the killer’s profile. But Kappen’s wife gave an alibi for him at the time, the BBC reported, drawing investigators off his trail. Kappen had died from lung cancer in 1990, meaning detectives would have to exhume his body in order to finally get an answer as to whether he was the "Saturday Night Strangler." A Mystery Solved After detectives exhumed Kappen's body, his DNA was matched to the samples found on the three victims in 1973, according to the BBC — confirming once and for all that he was the killer. "No words can describe the way we all felt, it was a huge relief," Geraldine's cousin Julie Begley told the outlet after Kappen was identified. "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." The murder mystery and the decades-long investigation that followed became the subject of a BBC true crime drama, Steeltown Murders, reviving interest in the case once again during the summer of 2023 when the series aired. "You know there are evils out there, but you never believe it will touch on you and yours,” Jean Hughes, Geraldine’s mother, told The Guardian after her daughter’s murder was solved in 2003. "When it does, it is a lifetime's sentence of hell. Now we can close the book on that hell forever."