Police investigating the murder of an off-duty special police constable are focusing on a man spotted running from the scene by neighbours.
Police searched the Wembley street for clues to the murder
Detectives say the sighting remains their strongest lead in the murder of Nisha Patel-Nasri, 29, stabbed in the leg outside her north-west London home.
The man, wearing a hooded top, ran from Sudbury Avenue, Wembley, towards Harrow Road following Thursday night's attack.
Police said the suspect was likely to have been heavily blood-stained.
Det Supt Julian Worker, in making an appeal for witnesses, said: "I want to reiterate there was an awful lot of blood spilt here.
"I believe the murderer would have been contaminated with that blood. They may have come home with bloodstained clothes. There may be people who have their suspicions. I'm asking them to come forward."
Mrs Patel-Nasri, who also worked as a hairdresser, was attacked shortly before midnight after she went outside in her night clothes.
Special constable Nisha Patel-Nasri
Police believe that she may have been investigating a noise outside her house.
"This sort of crime is just so atrocious and so horrendous. No loyalty even to a family member or to a friend is worthy of not coming forward," Det Supt Worker added.
"It's absolutely tragic that somebody who gives up their time and serves their community meets their death in such tragic and violent circumstances."
Police are considering whether a disturbance at the house a week earlier is connected to the murder.
UK's part-time police force, numbering more than 11,000
Made up of volunteers who work at least four hours a week
On duty they wear uniform and have full police powers
Duties may include foot patrols, house-to-house enquiries or giving evidence in court
Specials have been part of police force for 175 years
Last Saturday three men had tried to get into the house through the front door, but Mrs Patel-Nasri challenged them and they ran off.
Police are also examining the possibility that thieves were attempting to break into two expensive cars parked outside the house and belonging to her husband, who runs a limo hire company.
One neighbour told the BBC News website he had heard someone shouting "bachau, bachau" - meaning "help me" or "save me" - between 2330 BST and midnight on Thursday.
Other neighbours said they heard screams and found Mrs Patel-Nasri collapsed on the footpath.
They gave her first aid but she was later pronounced dead in hospital, Scotland Yard said.
A post-mortem showed she had died from a stab wound to her thigh.