The family of a girl murdered in 1965 said they still did not have "complete justice" despite a new inquest finding she was unlawfully killed.
Det Ch Supt Nick Wallen told Wakefield Coroner's Court he was "convinced" child killer Peter Pickering was responsible for Elsie Frost's death.
Pickering was close to being charged with her murder before he died in 2018.
Ms Frost's sister, Anne Cleave, said: "We've got a jigsaw puzzle that is almost complete, except for one piece."
The inquest heard the actions of the police force in 1965 meant Pickering was left free to abduct and rape an 18-year-old woman and murder another 14-year-old in Barnsley in 1972.
Det Ch Supt Wallen told the hearing Pickering was wanted for two sexual assaults at the time of Elsie's murder and a manhunt began the day after she was stabbed to death in Wakefield on 9 October 1965.
Officers put his home in Wombwell, Barnsley, under 24-hour surveillance but did not realise the person leaving and entering the house during that time was Pickering dressed as a woman.
Police said they were legally not allowed to enter the house and one crashed his car into a gatepost trying to intercept Pickering - who eventually fled after a car chase on 26 October.
Two months later another man, Ian Spencer, was charged with the schoolgirl's murder but was cleared in 1966 leaving the case cold for nearly 50 years.
Det Ch Supt Wallen told the inquest that police at the time believed Mr Spencer was the suspect and effectively closed the case after he was acquitted.
When the detective reopened the inquiry in 2015, he discovered that any forensic evidence had been destroyed in the intervening years.
However, they eventually made the link to Pickering and found two storage units containing documents written by the suspect over the previous 40 years which appeared to incriminate him in Elsie's murder.
The officer described Pickering as a "homicidal maniac" and told the inquest he was convinced the suspect was responsible for Elsie's murder.
Speaking after the verdict, he said: "When we commenced our investigation, we would never have anticipated we would have been able to identify a suspect for a crime then 50 years old.
"That man was, of course, Peter Pickering and I firmly believe that, if it had not been for his own death, he would have been charged and put before a jury to answer for Elsie's murder.
"While Colin and Anne sadly never had the chance to see Pickering charged and in court, I hope the conclusion of these proceedings may now bring them some comfort."
Coroner Kevin McLoughlin said he was unable to apportion blame in the inquest but described Pickering as "devious" and "dangerous".