The latest details of the investigation confirmed the views of senior officers that the killer of the television presenter was most likely an obsessive stalker or contract hitman.
Former partners of the 37-year-old were eliminated after extensive police inquiries in the United Kingdom since her murder in April.
But it is understood that police have not ruled out the possibility of other relationships still coming to light, and many more possible suspects are still to be interviewed.
"All the known boyfriends have been ruled out," said a police source.
Scotland Yard said there had been a good response to its appeal last month about the American-made Remington bullet that might have been specially customised by the assassin. The bullet had distinctive markings around the top of the cartridge case, which suggested the bullet had been tampered with.
Ammunition experts have indicated that the most likely reason was to remove some of the powder inside the case to muffle the sound of the gunshot.
The near 50-strong murder squad team is trawling through the "mass of information" it received after the appeal.
Miss Dando, who had been due to marry a gynaecologist, Alan Farthing, next month, was murdered on the doorstep of her home in Fulham, west London, on 26 April.
Detectives were satisfied that none of her "handful" of boyfriends in recent years had been at the scene of the murder.
Police were still tracing people whose names had been put forward as possible suspects by some of the hundreds of callers.
They still hoped to speak to the driver of a Range Rover seen near the scene who sped away shortly after the murder, but had never been traced by officers.Reuse content