Detectives believe the television presenter Jill Dando is highly unlikely to have been the victim of a contract killer.
The theory has been all but discounted after a surprising degree of co-operation the police have received from convicted hitmen.
The police officer in charge of the murder investigation disclosed that professional killers had telephoned and written to the Dando murder squad to offer tips and advice on the likely motive for the doorstep shooting.
They told the police that the killing was a "Mickey Mouse" affair, committed by an "amateur", and not the work of a professional hitman, said Detective Chief Inspector Hamish Campbell, the officer heading the murder hunt.
The 45-strong Dando squad now believe that the killer is most likely a man obsessed with the television presenter who had a knowledge of firearms, possibly through a gun club.
A profiler used by Scotland Yard believes the "skilled amateur" responsible is extremely unlikely to strike again.
A previous theory that the gunman was part of a team that included an unidentified man seen near by in a Range Rover has been all but discounted.
Scotland Yard officers have visited several hitmen in prison and examined the files of about 30 contract killings to find any similarities between their techniques and the way the television presenter was attacked.
Ms Dando, 37, was killed by a single shot to her head from a 9mm handgun outside her home in Gowan Avenue, Fulham, south-west London, on 26 April last year.
Det Ch Insp Hamish Campbell, the officer heading the murder hunt, said that a lot of criminals had contacted the inquiry to offer assistance. "Money is not their purpose, a lot say it's not right that the lady was killed."
As an example of the Dando killer's lack of experience, the criminals have highlighted how he left behind the spent cartridge case at the scene of the murder. A professional gunman would have taken it with him so that the police could not obtain any clues about the type of firearm and bullet used. The adapted handgun used is also considered an "amateur's weapon".
Among the criminals the police are understood to have visited in jail is Kevin Lane, known as "the Executioner". Lane was jailed for life in 1996 for killing Robert Magill, a wealthy car dealer, with a shotgun at close range as he walked his dog near Rickmansworth, Hertfordshire in 1994.
Det Ch Insp Campbell said he had "people in prison ringing up and other criminals ringing up and giving information that at times is just amazing". He added: "A lot of information is valid, but at the end of the day it has nothing to do with the Jill Dando murder."
About 1,000 clairvoyants and psychics have also written to Scotland Yard, naming Ms Dando's killer. They have all identified different people.
Det Ch Insp Campbell said his officers had examined between 20 and 30 contract killings that had taken place in the past decade.
They have also looked at 60 homicides in Britain since 1980 in which women have been killed with a handgun. None of the cases in either category is similar to the Dando killing.Reuse content