British police knew of claims that journalists at Rupert Murdoch's News of the World hacked into a murdered schoolgirl's mobile phone but they failed to investigate, the police watchdog said on Wednesday.
Britain's Independent Police Complaints Commission (IPCC) said Surrey Police in southeast England had done nothing about the allegations during their probe into the murder of 13-year-old Milly Dowler in 2002.
But the IPCC said it was unable to find out why nothing was done, because former senior officers at the force appeared to be suffering from "collective amnesia".
Revelations that the News of the World hacked into Dowler's voicemail messages, along with those of dozens of celebrities and public figures, sparked a huge public outcry that forced Murdoch to shut down the tabloid in 2011.
"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," said IPCC deputy chair Deborah Glass.
"We have not been able to uncover any evidence, in documentation or witness statements, of why and by whom that decision was made," she added.
"Former senior officers, in particular, appear to have been afflicted by a form of collective amnesia in relation to the events of 2002."
The phone-hacking scandal sparked three police investigations and a judicial inquiry into press ethics.
Testimony at the inquiry, led by judge Brian Leveson, revealed a close relationship between police and staff at Murdoch's British newspaper wing, News International.
Dozens of people have been arrested under Scotland Yard's probes into phone-hacking, computer hacking and the selling of stories by public officials.
On Wednesday a former Surrey Police officer became the 62nd person to be arrested under the probe into corrupt payments, Operation Elveden.
Prime Minister David Cameron's ex-media chief Andy Coulson, a former News of the World editor, is among those who have been charged in connection with the scandal, as is former News International chief executive Rebekah Brooks.
Former nightclub bouncer Levi Bellfield was convicted of murdering Milly Dowler in 2011, nine years after she disappeared on her way home from school.