Professor Alexis Jay's inquiry detailed gang rapes, grooming, trafficking and other sexual exploitation on a wide scale in the South Yorkshire town between 1997 and 2013.
Professor Jay said given the information available to agencies by April 2005, "nobody could say 'I didn't know'."
Her comments came as Home Secretary Theresa May, on a visit to Dumfries, heaped further pressure on South Yorkshire police and crime commissioner Shaun Wright by appearing to join calls for him to quit after a shocking report into child abuse in the city.
He was the council cabinet member responsible for children's services in the English town from 2005 to 2010, in the middle of a 16-year period when, according to the report, 1,400 youngsters suffered widescale sexual exploitation including gang rapes, grooming and trafficking.
He has so far refused to quit, saying he had no knowledge of the "industrial scale" of child abuse at the time.
Ms Jay, who was educated in Edinburgh and is a former director of social work at West Dunbartonshire Council, said: "Part of my remit was to identify what information was available to key people in positions of influence throughout that time.
"And there was certainly a very great deal of information available from an early stage; indeed from at least 2001, both through a youth project which did outreach work with these young victims and children's social care.
"But also because there were at least three key reports which were made available to the agencies concerned whose conclusions couldn't have been clearer.
"Then finally members of the council had seminars organised at which the detail of the youth project and indeed some of the other material, was included in that.
"Names of potential perpetrators, car registration numbers, a very great deal of detail. Really by April 2005, it seemed to me that nobody could say 'I didn't know'."
She said the exploitation covered by the report was "at the worst end of seriousness".
Ms Jay said: "I have spent decades looking at complex cases of child protection and I have never encountered such brutality and such abuse."
She cited a number of possible reasons why authorities did not get to grips with the issue of child sexual exploitation.
She said: "There were also concerns that the priority in child protection at the time was younger children and that these mainly girls were making lifestyle choices."