Police have admitted they may never catch the murderer of BBC presenter Jill Dando.

Metropolitan Commissioner Sir Paul Condon said the time might come when "there is nothing more to be done".

But on Saturday night a spokeswoman for Scotland Yard stressed the Dando case "remained solvable".

She said: "There are many lines of inquiry still to pursue which we will continue to do vigorously."

Ms Dando, 37, who presented the BBC's Crimewatch programme, was shot in the head at close range with a semi-automatic 9mm handgun outside her home in Fulham, west London, on 26 April.

Despite a massive murder inquiry, police have not pinpointed a motive or a suspect.

Sir Paul, in an interview with the Daily Mail, said: "When - if and when - the team reaches the point where they have given it their best go and there is nothing more to be done, then we have to say that.

"But we are certainly nowhere near that yet. We wouldn't close the book - we'd keep the file open.

"There are a small amount of stranger murders that remain unsolved. Because everyone knew and loved Jill, all the focus is on her case."

Sir Paul, who became friends with Ms Dando through helping on Crimewatch, added: "But there are others - such as a girl killed walking her dog in the West Country. For the detective that sort of stranger murder is the hardest to solve."

Meanwhile detectives have revealed that a third attempt to gain access to Ms Dando's private documents was made shortly before her murder by an unidentified man.

Police have previously uncovered two similar attempts, adding strength to one of the lines of inquiry they are following - that she was killed by an obsessed stalker.

Last month, police revealed that in the days leading up to her death, a man had pretended to be her brother in an attempt to get her electricity bill moved to his name.

On another occasion, a man contacted BT, giving Miss Dando's ex-directory telephone number and address and asking for her line to be changed to his name.

A Scotland Yard spokesman said: "Police previously said that someone claiming to be Miss Dando's brother asked for her electricity bill and BT bill to be changed to his name at the same address at 29 Gowan Avenue (Miss Dando's address).

"Police now know that the same thing happened on a third occasion when a man called and asked for another of Miss Dando's bills to be changed to his name.

"Police are linking the three incidents but are not discussing it further."

Earlier this week police confirmed that detectives hunting her killer are to fly to the US this month to seek advice from ballistics experts about the weapon used to kill her.

Police believe the most likely motive behind the shooting remains either the obsessed behaviour of a stalker or an underworld contract killing.