Senior police officer 'in no doubt' who killed Elsie Frost in Wakefield
Detective chief superintendent Nick Wallen, of West Yorkshire Police, told Wakefield Coroners' Court the original investigation was "packed up" after the prime suspect Ian Spencer was acquitted.
DCS Wallen said he felt senior investigators in the 1960s "had their men and went after him", despite a lack of evidence.
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Hide AdHe said that Mr Spencer was not legally represented at the original inquest into Elsie's death in January 1966.
He added that witness statements placed Mr Spencer in four different places at the same time.
After Mr Spencer was formally acquitted, detectives from Scotland Yard dropped the case and went back to London, DCS Wallen told the inquest.
He said: "Cases like this are the reason we have the Police and Criminal Evidence Act and that all people in police custody are offered free, independent legal advice if they wish to have it."
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Hide AdThe court heard that detectives reviewing the case in 2015 found a telegram from the Metropolitan Police force to officers in Wakefield identifying Peter Pickering as a suspect.
Pickering, who had previously been jailed for sexual assault against young women, had been released a year before Elsie's murder, the court heard.
DCS Wallen said Pickering was "on the run" from police who were investigating two serious sexual assaults against young women in different parts of Yorkshire in the months before Elsie was killed.
He added: "We can place Pickering in Wombwell, not far from Wakefield, in the days building up to Elsie's murder.
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Hide Ad"He was in an absolute rage because an alibi provided for him had been retracted by a girlfriend
"He wrote a very threatening letter in which he threatened to kill someone."
The six-page letter, shown to the court, read: "Now no doubt you'll read of me in the papers as soon future date that fate will decide.
"I shall go down in flames this time.
"If I don't kill myself - but this I doubt because in that respect I am a coward - I shall have to take someone with me when I go.
"So what now? To join the Devil.
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Hide Ad"I'm no longer me anymore, now I'm really going to get good and bloody nasty."
DCS Wallen said: "I would say, given everything else we know, we know have a letter written by him three or four days before Elsie is killed making a threat to kill someone.
"We know he has referred to himself as a Jekyll and Hyde character in his letters.
"He has access to premises close to the crime scene. We have ample evidence that he knew Wakefield very well indeed."
The inquest continues.