The police watchdog has found that former senior officers at Surrey Police failed to act on evidence of the alleged hacking of schoolgirl Milly Dowler's phone during their 2002 investigation of her murder.
Officers at all levels of the investigation knew that an allegation of hacking had been made against the News of the World but did nothing despite suggestions that a crime had been committed, the Independent Police Complaints Commission (IPCC) said.
But the watchdog added it had not been able to discover why nothing was done, adding that senior officers appeared to be suffering from a "form of collective amnesia''.
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.
IPCC deputy chair Deborah Glass said: "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.
"Phone hacking was a crime and this should have been acted upon, if not in 2002, then later, once the News of the World's widespread use of phone hacking became a matter of public knowledge and concern.
"Our investigation has heard from officers and former officers from Surrey Police who have expressed surprise and dismay that it wasn't investigated.
"We have not been able to uncover any evidence, in documentation or witness statements, of why and by whom that decision was made - former senior officers, in particular, appear to have been afflicted by a form of collective amnesia in relation to the events of 2002.
"This is perhaps not surprising, given the events of 2011 and the public outcry that the hacking of Milly Dowler's phone produced.''