The truth about how murdered schoolgirl Milly Dowler's phone messages were deleted may never emerge, the Leveson Inquiry heard today.
The News of the World admitted hacking the 13-year-old's mobile phone but it remains unknown whether two missing messages were deleted deliberately or were removed from her message box automatically.
The inquiry into press standards has heard that Milly's mother Sally phoned her daughter repeatedly in March 2002 after she vanished while walking home in Walton-on-Thames, Surrey.
The teenager's voicemail message was a generic automated response when her message box was full but when a message had been deleted the greeting reverted to her personal greeting.
The Dowlers previously told the inquiry they were given "false hope" by hearing the change of greeting - thinking their daughter might still be alive and had wiped a message.
Prime Minister David Cameron set up the Leveson Inquiry last summer amid claims, first reported in The Guardian, that the now defunct Sunday paper not only hacked Milly's phone but also deleted messages.
But in December the Leveson Inquiry was told police actually thought it was "unlikely" the News of the World was responsible for the deletions.
Today the inquiry heard that because of incomplete phone records and the passage of time, it may never be possible to find out exactly what had happened to the messages.
The inquiry heard that messages on Milly's phone would be deleted after 72 hours.
Metropolitan Police detective chief inspector John MacDonald, investigating what happened, said in a statement: "In summary, we cannot conclusively say whether any voicemails were manually deleted.
"However, there do appear to have been two messages missing that should have been present when Surrey Police carried out their second recorded download on April 17.
"It is not known why that happened and it will not now be possible to provide an explanation.
"It is not anticipated that any further clarity will be obtained on this issue."
News International, owner of the News of the World, apologised again for hacking the phone which they said was done before the moment of "false hope" the schoolgirl's parents experienced.
In a statement read by David Sherborne, counsel for hacking victims, Mr and Mrs Dowler praised the Guardian for its role in uncovering the tabloid's hacking of phones.
They said: "If Surrey Police had prosecuted this activity in 2002 then the position would have been very different and perhaps countless others might also have avoided having their private messages hacked into by the News of the World.
"Police neglect and deference meant that it took the relentless efforts of one journalist to uncover what the police knew had gone on, and whilst we would never have wished to have been thrust into the middle of this extraordinary scandal on top of what we have already had to deal with as a family, we continue to have faith that his efforts and the efforts of the inquiry and Operation Weeting will have a lasting positive impact."
In a statement read to the inquiry, Gill Phillips, head of legal services at The Guardian, said the newspaper's story of July 4 2011, disclosing that Milly's phone had been hacked, was "based on multiple sources and their state of knowledge at the time".
"Our error - as we acknowledged and corrected last December - was to have written about the cause of the deletions as a fact rather than as the belief of several people involved in the case. We regret that.
"After five more months of intensive inquiry, the police have found that the passage of time and the loss of evidence means that 'reaching a definitive conclusion is not, and may never be, possible'."