The parents of murdered teenager Georgia Williams have described the findings of a serious case review into her death as "an embarrassment" to the police.
Steve and Lynnette Williams said the report showed their 17-year-old daughter's death could have been prevented and that her killer, Jamie Reynolds, was "a murderer in the making".
Mr and Mrs Williams also called on West Mercia Police to publish a further report prepared by Devon and Cornwall Police which they said highlighted mistakes "10 times worse" than those identified by the serious case review.
The independent Devon and Cornwall report, given to the West Mercia force in March, led to misconduct proceedings against four officers and one civilian staff member who face a sanction stopping short of dismissal.
Georgia's father told a press conference in Telford, Shropshire: "It (the Devon and Cornwall report) should be released because it gives all the answers to the questions that remain."
Holding up a copy of the serious case review, Mr Williams, who has read the outside force's report, said: "This is an outline and a suggestion of what went wrong.
"The Devon and Cornwall (review) confirmed what went wrong."
Mr Williams, a detective with West Mercia Police, said of the serious case review: "We cried when we read this.
"And we cried even more when we read the Devon and Cornwall (review) and the context of it, and the answers to why officers didn't do what they should have.
"In our eyes it's 10 times worse and it would be embarrassment to the police, like this is an embarrassment to the police.
"But I think it should come out because it gives you the answers. This is only half the story. It's like reading a novel and closing it halfway through and not knowing the end."
Reynolds, who is serving a whole life sentence, lured Georgia into going to his home in Wellington, Shropshire, in 2013 under the guise of a photoshoot.
The then 23-year-old killed Georgia in a meticulously planned trap, hanging her from a length of rope attached to the loft hatch.
It later emerged that Reynolds had come to the attention of police five years before the murder when he trapped another 16-year-old girl at his home and grabbed her around the throat in a "bizarre, potentially serious and unprovoked attack".
The discretionary serious case review by West Mercia's public protection management board raised "serious concerns about the quality of the investigation" in 2008, adding the police inquiry into the incident was "narrow in its perspective" and seemed "aimed at ensuring a speedy resolution".
In a statement released with the case review, Mr and Mrs Williams said: "Having lost Georgia to pure evil, we cried when we read this report and the failings of all agencies involved because it was so obvious that Reynolds was, if not one already, a murderer in the making.
"Georgia's death could have been prevented - learn if nothing else."
The report reveals that Reynolds also launched an "unprovoked and violent" attack on a woman after luring her to his home on the pretence of helping with a school project.
Officers treated the incident as an assault and gave Reynolds a final warning.
Weeks later, images of schoolgirls with nooses drawn around their necks were found in Reynolds's bedroom and handed to police by his family.
A doctor then assessed Reynolds as being a significant risk to others, "on the basis that he seemed to have progressed from viewing sexually violent pornography to acting upon it" when he attacked the teenager.
Eight agencies were involved after the attack, but the report says there was a "confused and unco-ordinated approach to the case" among the people looking after Reynolds, including mental health services, police, children's services and the probation trust.
"The work undertaken in respect of Reynolds following the offence in 2008 was disjointed, lacked focus, did not include a clear multi-agency risk management plan and was restricted to single agency, short term perspectives," the report concluded.
Three years later, Reynolds was again reported to the police for reversing his car into that of a girl who had spurned his amorous advances.
The report said: "When Reynolds came to the attention of the police in 2011 for what was clearly an offence with some links to the 2008 offence, no link was made to the information existing within police files.
"Following the murder of Georgia, the matter was investigated further and at that point it became clear that Reynolds' inappropriate behaviour in relation to his colleague had been persistent and increasingly obsessive.
"His actions at the time of the incident and immediately following it had a number of parallels with the 2008 offence.
"It is clear that had this matter been looked into in more detail it would have highlighted Reynolds' ongoing and developing behaviour and the need for serious concern about the risks he posed."
The report made five recommendations which relate to improving the sharing of information between agencies, as well as raising concerns nationally about the lack of guidance for staff when dealing with potentially serious offending at the point of transition from youth to adult.
It added: "It is inevitable that awareness of these shortcomings will cause distress to the parents of Georgia and it is of general concern that, for whatever reason, the investigation into the case was not as thorough as might reasonably have been expected."
Chief Constable of West Mercia Police, David Shaw, said: "We could and should have done better. It is as simple as that.
"We let Georgia down. We let Steve and Lynnette down. And as you'll see in the report - some other young people, we let them down as well.
"What that report reflects is a whole series of things that the police and other agencies can do to improve, and make sure the failings do not happen again."