An investigation into three ex-senior officers who led an inquiry into child sexual exploitation has been discontinued by the police watchdog.
It follows a review by the Independent Office for Police Conduct (IOPC) into the decision by Greater Manchester Police (GMP) to close down the inquiry in 2005, a year after it was launched.
The IOPC said there was no indication the officers had breached standards.
GMP said the force's practices had "substantially changed" since then.
The IOPC investigation was set up after Greater Manchester Mayor Andy Burnham commissioned a report following the 2017 BBC documentary, The Betrayed Girls.
It focussed on the death of 15-year-old Victoria Agoglia in 2003, who was in care and had reported being raped and injected with heroin by a 50-year-old man.
She died of a drugs overdose two months later on 29 September, 2003.
The documentary also featured GMP's subsequent investigation into child sexual exploitation in south Manchester called Operation Augusta.
Operation Augusta identified at least 57 victims and 97 potential suspects, the 2020 report found.
But it added that senior officers at GMP had chosen to under-resource the investigation and a decision had been made to close it down in 2005.
Following the publication of the review, GMP made a referral relating to three former senior officers to the IOPC.
The IOPC said it had "obtained a significant amount of evidence" as part of its investigation.
But it added it was "unable to determine who took the final decision to close Operation Augusta in July 2005, nor the rationale for doing so".
"Challenges faced included the passage of time; a lack of available records of meetings and decisions taken at that time; and the fact some former GMP-employed police witnesses were either unable or unwilling to engage with our investigation," the IOPC added.
Outlining the decision to discontinue the investigation, the police watchdog said it had "determined there was no indication any of the three former officers had acted in a way that may have breached the standards of professional behaviour".
But Mr Burnham said he was "very disappointed with this inconclusive and inadequate investigation", adding victims had been "sadly been let down again".
It was "now a matter for the Home Secretary", he said.
"She needs to say whether she finds this investigation acceptable and, if not, what she intends to do about it."
Steve Noonan, the IOPC's director of major investigations, said that the watchdog had "identified several areas of potential learning for GMP to consider".
"[We] now await further information on how practices have changed since 2005, which will inform our decision on whether to issue any statutory recommendations."
Deputy Chief Constable Terry Woods, from GMP, said the force's approach to tackling and preventing child sexual exploitation was "now thankfully entirely different".
"We have a dedicated child sexual exploitation (CSE) unit who are committed to pursuing perpetrators and seeking justice for victims of CSE, regardless of the passage of time."
He added the force would continue to engage with the IOPC on how practices had substantially changed since then and would "fully consider" any recommendations they put forward.