The police watchdog has defended ending an investigation into how three senior officers led an inquiry into child sexual exploitation.
The Independent Office for Police Conduct (IOPC) said it could not find out who halted the inquiry in Manchester.
Greater Manchester Mayor Andy Burnham said the IOPC investigation was "inadequate".
The IOPC said its probe was "extremely thorough".
The IOPC investigation was set up after Mr Burnham commissioned a report following the 2017 release of BBC documentary The Betrayed Girls.
It focused 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 drug overdose two months later on 29 September 2003.
The IOPC said it never found out why the year-long investigation into child sexual exploitation in south Manchester was halted and some former Greater Manchester Police (GMP) members did not engage with its work
On Thursday it said it had discontinued its own investigation and had found no evidence GMP breached any professional standards
Mr Burnham criticised the IOPC, saying he was "very disappointed with this inconclusive and inadequate investigation", and victims had "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, IOPC director of major investigations, said: "Our investigation was extremely thorough and we reached our conclusions based on an amount of evidence, above and beyond what had previously been considered in relation to this matter.
"We make every effort to obtain information from witnesses we feel may be able to assist with our inquiries. Unfortunately, where individuals are either unable or unwilling to cooperate with an investigation, this will inevitably affect the evidence we are able to gather.
"Transparency is essential for public confidence and that is why we chose to highlight this, as well as other challenges faced during what was a complex investigation, in our publication."
The IOPC said there was no indication the officers had breached standards.
GMP said the force's practices had "substantially changed" since then.
A Home Office spokesman said: "The Home Secretary has no powers to review the IOPC's decisions or to overturn them.
"The government is currently considering recommendations for a broader duty of candour for public bodies and authorities and we will publish our response in due course."