A special constable was stabbed to death as she stood outside her home in her night clothes.
Mrs Patel-Nasri was stabbed outside her home
Nisha Patel-Nasri, 29, was off duty when she was attacked in Sudbury Avenue, Wembley, in north-west London.
Neighbours, who alerted police after they heard her scream, gave her first aid but she was later pronounced dead in hospital, Scotland Yard said.
A link between her murder on Thursday night and an attempted burglary at the house is being investigated.
Three men tried to get into the house through the front door, but ran off when she challenged them on Saturday night.
A man wearing a hooded top was seen running towards Harrow Road after the murder.
A post-mortem examination showed Mrs Patel-Nasri died from a single stab wound to the groin.
The hairdresser was alone at the house she shared with her husband, Fadi Nasri, before the attack took place.
Appealing for witnesses at the murder scene Det Supt Julian Worker said: "Clearly she suffered a fatal wound to her leg.
"What we believe has happened is that she has come out of her house in her night dress and has dealt with something in the street."
"I want to reiterate there was an awful lot of blood spilt here. I believe the murderer would have been contaminated with that blood."
Mrs Patel-Nasri had confronted burglars at her home days earlier
Det Supt Worker said two cars of high value were parked outside at the time.
"Those vehicles may have been the cause of attention to somebody," he said.
BBC reporter Daniela Ralph said police were not making any connection between Mrs Patel-Nasri's role as a special constable and her death.
Scotland Yard described her as a "tremendously popular" member of the Metropolitan Police's Special Constabulary in the London Borough of Brent.
Met Police Commissioner Sir Ian Blair said: "It is a tragedy that a young woman who willingly gave up so much of her valuable free time to work as a police officer in her local community should lose her life in this terrible way.
"Nisha was, I'm told, tremendously popular with her colleagues, who regarded her as a real team player."
A section of Sudbury Avenue has been cordoned off while forensics teams carry out a search of the area.
Specials have the same powers as regular police. They assist officers, carrying out their duties in their spare time.