The police watchdog is investigating three former senior Greater Manchester Police (GMP) officers who led a probe into child sexual exploitation.
The Independent Office for Police Conduct (IOPC) will look at how the 2004 investigation was supervised.
It follows a critical review in January that centred on the death of 15-year-old Victoria Agoglia in 2003.
Two of the officers have since retired and a third is currently working for another organisation.
Greater Manchester Mayor Andy Burnham commissioned a report after the 2017 BBC documentary The Betrayed Girls.
It focussed on the death of Victoria – who was in care and reported being injected with heroin by a 50-year-old Asian man – and GMP’s subsequent investigation, Operation Augusta.
It was set up to tackle “the sexual exploitation throughout a wide area of a significant number of children in the care system by predominantly Asian men”, the report said
It had identified at least 57 victims of child grooming and 97 potential suspects but “very few” faced justice.
Senior officers at GMP chose to under-resource the investigation, the report to Mr Burnham said, and a decision was made to close it down in 2005.
‘Investigate individual conduct’
IOPC director of major investigations Steve Noonan said: “This has been a lengthy but necessary process and it has taken some time to gather the information required to make a decision whether it was necessary to investigate and who should conduct the investigation.
“The mayoral review raised many questions about the actions of those involved in Operation Augusta.
“We have now decided it is necessary to investigate the individual conduct of these three former GMP officers who were all involved in either supervising or setting the direction of Operation Augusta.
“Though the content of the mayoral review has been analysed as part of our decision to begin an investigation, it was not a specific investigation into the actions of the officers involved.
“We now need to establish and examine all the available evidence.”