Officers ignored repeated warnings from the ex-girlfriend of paedophile rock star Ian Watkins that he had sent her an indecent image of a child, the Independent Police Complaints Commission (IPCC) has found.
Joanne Mjadzelics took a laptop three times to Doncaster Police Station between March and May 2012, but South Yorkshire Police officers failed to examine it.
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 until later but it sent a document in error to the Yorkshire Police 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 baby.