Detectives hunting the killer of TV presenter Jill Dando are to fly to the US to seek expert help on identifying the weapon used to kill her.

Ms Dando was shot dead at close range on the doorstep of her home in Fulham, west London, in April.

Distinctive markings on the bullet cartridge used in the shooting have been identified as a potentially vital clue in tracking down the killer.

A Scotland Yard spokesman said the officers would spend "as long as was necessary" in the US, seeking expert assistance on ballistics matters.

The cartridge had six distinctive marks, leading gun experts to believe it could have been tampered with, possibly to reduce the amount of gunpowder and deaden the noise of the gunshot.

This has led them to believe Ms Dando's killer may have been someone with an intimate knowledge of firearms.

Last month it emerged that police think Miss Dando was the target of a stalker in the months leading up to her murder, adding extra weight to theories that an obsessed fan was behind the killing.

Detectives revealed the bizarre behaviour of a number of men in the months before her death, including one who claimed to be her brother and tried to have 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.

However, Scotland Yard would not comment on reports that detectives would also be talking to experts on stalking while in the US