Police officers knew about claims that News of the World reporters had hacked murdered schoolgirl Milly Dowler's phone more than 10 years ago but did nothing, the police watchdog has found.
Officers at "all levels" of Surrey Police were aware of the allegations during the 2002 murder investigation, the Independent Police Complaints Commission (IPCC) said.
But the watchdog added it had not been able to discover why the force failed to act, adding that senior officers were suffering from a "form of collective amnesia".
Mark Lewis, the Dowler family's lawyer, said they could not comment due to other legal proceedings - but he did say "it was not the finest hour for the Surrey Police".
The findings follow an investigation into the conduct of two senior officers, deputy chief constable Craig Denholm and temporary detective superintendent Maria Woodall.
Surrey Police said it had taken "management action and issued words of advice" to both officers, although the IPCC concluded neither had a case to answer for misconduct.
"We will never know what would have happened had Surrey Police carried out an investigation into the hacking of Milly Dowler's phone in 2002," IPCC deputy chair Deborah Glass said.
Former nightclub bouncer Levi Bellfield was convicted of Milly's murder in June 2011, some nine years after she vanished as she walked home from Walton-on-Thames station. Ater his trial, then Surrey Police chief constable Mark Rowley set up Operation Baronet to look into reports that the force was aware in April 2002 that the News of the World had allegedly intercepted her voicemail.
Surrey Police Authority and Surrey Police referred Mr Denholm and Ms Woodall to the IPCC in June 2012 in light of evidence arising from Operation Baronet.
The now defunct News of the World admitted hacking Milly's mobile phone but it remains unknown whether two missing voicemail messages were deleted deliberately.