Lostprophets singer Ian Watkins was shopped to police about his child sex abuse, but nothing was done.
A former girlfriend of the pop star claimed she told South Wales Police the 36-year-old was a danger to children five years ago.
Joanna Majic, 38, who had an on-off relationship with the frontman, said she was not believed by officers when she came forward in 2008.
This week, Watkins admitted a string of child sex abuse charges which were described by police as "the most shocking and harrowing child abuse evidence seen." He had previously "furiously denied" the allegations since being arrested along with two women last year.
A computer containing banned images and video which belonged to the Welshman was protected by the password: 'if***kids.'
Majic, who said she and Watkins were a couple for a while, told how the singer believed he was above the law because of his celebrity status. He revealed to her his depraved appetite during a series of sexual encounters.
"Everything changed for us when he confided in me that he wanted to abuse children in 2008," she told the Mirror.
"I went straight to South Wales Police then and made a complaint but they ignored me and said I needed evidence.
"I warned them this man was a danger to young kids but I think they thought I was just a trouble maker.
"I couldn't believe how indifferent the police were, it was like they were taking his side and dismissing me as some stalker.
"I told Ian I had gone to the police and we argued but he was so cocky, he did not think the police would touch him and he was right.
"It was then I realised I had to turn off any feelings I had for him and turn on my escort mode.
"I had to treat him as any other client and try and get as much evidence as I could while pretending to be whatever it was he wanted me to be.
"I was determined to save more children from this monster."
Chief Superintendent Tim Jones, head of South Wales Police's Professional Standards Department, said: "South Wales Police will fully support the Independent Police Complaints Commission's Independent Investigation and are committed to responding promptly to its findings."