The knife used to murder special constable Nisha Patel-Nasri outside her home four months ago has been found.
Mrs Patel-Nasri had been a special constable for three years
Police said the knife, which was missing from Mrs Patel-Nasri's kitchen, was used to stab her in the leg.
They have also identified a car seen near her home in Wembley, north-west London, which they think was involved.
Police said DNA of her suspected killer found on the knife was not good enough to match with the national database, but may be used to eliminate suspects.
Mrs Patel-Nasri, a hairdresser who worked as a part-time police woman, had been out for a meal that night with her husband Fadi to celebrate their third wedding anniversary.
The 29-year-old is thought to have either gone outside to investigate noises shortly before midnight on 11 May, or found someone in her house.
She may have carried the knife with her - a John Lewis "Evolution" cook's knife - only for it to have been used against her.
Police have not revealed how or where it was found.
Future advances in DNA techniques may allow the profile to be developed further, but it is not yet sufficient to run against all of the two million samples on the database.
But along with the car, a light coloured Audi A4 saloon, seen close to the murder scene about an hour before the stabbing, detectives believe they have the evidence to build a strong case.
Police have released CCTV footage of a potential witness
Det Supt Julian Worker said the "suspicious behaviour" of those driving the car led him to believe it was directly involved.
It was spotted in Harrowdene Road, which runs parallel to Sudbury Avenue - where Mrs Patel-Nasri lived - twice on the night of the stabbing.
The car was filmed by CCTV cameras in the hour before the murder, turning out of the road, then back again, waiting at the side of the road for a matter of seconds and trying to edge forward at traffic lights.
Detectives believe it may have pulled over to pick up a suspect, seen moments after the murder.
It had a number of distinctive features including a blemish on the roof and a licence plate light which was not working that night.
They have also released a still CCTV image of a man they believe may be a potential witness.
An appeal will be made on the BBC's Crimewatch programme on Tuesday night for help catching the killer.
"There is someone out there that has killed someone and I am asking for people with information to come forward," said Det Supt Worker.
"There is a DNA profile which will prove or disprove somebody's involvement in this offence."
Four months on, detectives still do not know for certain why Mrs Patel-Nasri was killed.
Days before there had been an attempted burglary at her home and police have not ruled out a link between the two incidents.