Officers who would have faced gross misconduct hearings over failures to investigate allegations about paedophile rocker Ian Watkins will face no action as they have retired, the Independent Police Complaints Commission said.
They ignored repeated warnings from the ex-girlfriend of the Lostprophets singer that he had sent her an indecent image of a child.
Joanne Mjadzelics took a laptop three times to Doncaster Police Station between March and May 2012, but it was only on the last time that South Yorkshire Police officers looked at it and concluded the image was a close-up of an adult.
The IPCC has investigated the force and found their inaction may have left a child at risk for several months.
The watchdog has concluded that three officers would have a case to answer for gross misconduct, but as each has retired following 30 years' service, no further action can be taken.
One officer will face a misconduct hearing over allegations of an inappropriate remark being made to Ms Mjadzelics.
The IPCC had not been due to release its findings next week, but South Yorkshire Police sent a document in error to the Yorkshire Post newspaper, which then published details about the inquiry on its website.
Watkins, 40, was jailed for 29 years in 2013 for a string of serious sex offences, including the attempted rape of a fan's baby.
Ms Mjadzelics was cleared of possessing indecent images of children in January 2015, having told Cardiff Crown Court she was trying to entrap the depraved singer.
In March 2012, South Wales Police had asked colleagues in South Yorkshire to assist with allegations made by Ms Mjadzelics, who was living in the force area at the time.
Initially, the Safer Neighbourhood Team was tasked to respond, rather than specialists from the Public Protection Unit (PPU).
That led to a Pc with no training in child sex abuse investigations or in handling evidence in computer-related offences conducting an initial meeting at the police station.
The IPCC said the constable did not view any image, and there was no determined effort to ascertain whether Ms Mjadzelics had evidence of child sexual abuse.
Two months later, a constable from the PPU was told to seize the laptop and to take a statement from the rocker's ex-partner, but this did not happen.
When they viewed the alleged indecent image of a child, officers believed it was a close-up of an adult.
This was on the third time she had visited the police station.
The IPCC also said a police constable involved could have made inappropriate remarks about why Ms Mjadzelics was making the allegations about Watkins.
The laptop had been destroyed prior to Watkins' arrest.
Ms Mjadzelics subsequently complained that South Yorkshire Police did not examine the computer properly, and also that a female officer had used inappropriate language towards her on one occasion.
IPCC Commissioner Jan Williams said: "Having taken into consideration the nature and seriousness of Ms Mjadzelics' allegations against Watkins, the inaction of some South Yorkshire police officers involved may have placed a child at risk of further abuse for several months.
"The evidence suggests there was a general view among officers at Doncaster that Ms Mjadzelics was not to be taken seriously, and consequently enquiries were not progressed as they should have been.
"It is concerning that a neighbourhood police constable without specific training or support, rather than an officer from a specialist team, was expected to view and make judgement on a potential image of child sexual abuse."