Police investigating the murder of special constable Nisha Patel-Nasri are trying to trace a couple who may have seen a man running from the scene.
Mrs Nasri was fatally stabbed outside her home
Mrs Nasri, 29, was stabbed to death on the doorstep of her north-west London home just before midnight on Thursday.
Her brother, Katen Patel, who was due to marry on Sunday, has now cancelled the ceremony. He said his sister had been "so excited" about the event.
The couple who police want to trace were white and casually dressed.
Mr Patel, 33, said his sister had "practically organised" his side of his wedding.
Earlier, Mrs Nasri's husband, Fadi, paid tribute to his wife, saying she was "extremely hard-working, bubbly and always on the go."
Fadi and Nisha Patel-Nasri at their wedding with brother Katen
The couple had been celebrating their third wedding anniversary on the day she was killed.
Mrs Nasri's brother said: "Nisha will be hugely missed by everyone that knew her, especially by Fadi and myself. Things will never ever be the same," he added.
Detectives appealing for the young couple who may have seen a suspect fleeing the scene, say they were walking between Sylvester Road and Sudbury Avenue, Wembley, where the incident took place.
The man, who was spotted by neighbours, was said to be wearing a hooded top and ran from Sudbury Avenue towards Harrow Road.
Police say the suspect was likely to have been heavily blood-stained.
Det Supt Julian Worker said: "I want to reiterate there was an awful lot of blood spilt here."
The sighting remains the strongest lead in the police inquiry into the murder.
Mr Nasri said his wife was "a helpful and forgiving woman who was proud to wear a police uniform".
Mr Nasri, 32, who runs a limousine firm from his home in Wembley, north London, said his wife, a full-time hairdresser, had been a special constable for three years and "loved it".
"I was very proud of her and when she said she wanted to do it, I encouraged her but I had my worries for her safety.
Mrs Nasri was attacked shortly before midnight after she went outside in her night clothes.
Police believe that she may have been investigating a noise outside her house.