An off-duty special constable may have been murdered with a kitchen knife she carried while investigating a noise outside her home, police have said.
Police showed the type of knife they want to trace
Nisha Patel-Nasri, 29, bled to death after being stabbed in the leg in Wembley, north-west London, on Thursday night. The weapon has not been found.
Officers said they still wanted to talk to a couple seen in the area.
They also want to trace a man seen heading towards East Lane who did not look back despite the victim's screams.
The knife police want to trace is a John Lewis Evolution stainless steel knife, which is made from one piece of metal and does not have a wooden handle.
Mrs Patel-Nasri was stabbed in the leg
Police said the blade was about 20cm long with the words John Lewis embossed on it, and it would have been covered in blood.
Another man, who was wearing a hooded top and ran in the direction of Harrow Road, is also being sought.
On the night of her death Mrs Patel-Nasri had been out for a meal with her husband Fadi, 32, to celebrate their third wedding anniversary.
Mr Nasri then headed to play snooker but was called back after the attack on his wife. She was stabbed in the leg and bled to death.
Det Supt Julian Worker, leading the investigation, said of the man seen walking away from the murder scene: "The truth of the matter is Nisha is screaming and he doesn't look back.
"Everybody's attention may have been drawn to the man running away instead of the man walking away. He may not have been involved but I definitely need that person to come forward."
Officers are keeping an open mind about the motive for the killing.
Scotland Yard confirmed Mr Nasri had been threatened earlier in the year in connection with his limousine business.
High value cars were often parked outside their home and there has been speculation that these may have attracted thieves.
Possible links with an attempted burglary at the house on 6 May are also being looked into.
On Sunday, Mr Patel and his wife's brother Katen Patel, 33, appealed for witnesses to come forward.
Mr Patel cancelled his wedding, which was due to take place this weekend, after his sister's death.