The failure by British police to investigate the hacking of murdered schoolgirl Milly Dowler's phone was "unacceptable" and "a matter of deep regret", an internal investigation has said.
Surrey Police officers may not have pursued the News of the World about information gleaned from a message left on her voicemail "because of the potential consequences", a report said.
But it was more likely the matter was not recognised as illegal as phone hacking was not a widely used term at the time of her disappearance in 2002 and the scale of the defunct tabloid's use of it was unknown at the time, it added.
The report also said all effort and energy was focused by investigating officers on finding Milly, and other matters were either not considered or prioritised.
It added that no evidence was found to suggest there was any collusion between Surrey Police and the News of the World, which ceased publication in 2011.
Milly, 13, was snatched from the street while on her way from school to her home in Walton-on-Thames, Surrey, in March 2002.
Serial killer Levi Bellfield is serving a whole-life tariff after being found guilty of abducting and killing her following a trial at the Old Bailey.