New CCTV images have been released in connection with the 2017 Custom House killing of Ahmed Deen-Jah - whose brother was also fatally stabbed last April.
Ahead of the fifth anniversary of Ahmed's death, a reward of up to £20,000 is being offered for information that leads to his killer being caught.
Ahmed, who was 24, died after being stabbed in the heart in what police say was an unprovoked attack at a convenience store in Freemasons Road on the afternoon of April 2, 2017.
Following his death, Ahmed's father Abubakarr Jah said: "Ahmed was a beautiful, kind boy. He wanted to do something with his life.
"Now we have lost him. He didn’t deserve for this to happen.
"Young people need to put down their knives and stop the violence. It is destroying families and communities."
Police say three people, all aged 23, are currently released under investigation and another three were released with no further action in connection with the incident.
But five years on, nobody has been brought to justice for the killing.
Detectives are renewing appeals for information and have released images of a person they would like to identify as part of the investigation.
Around 3.20pm on the day of his death, Ahmed had gone into the shop and bought a cigarette lighter.
He left the store but then returned around 30 seconds later, followed by a person wearing dark clothing, a balaclava and gloves.
The suspect chased Ahmed around the shop before attacking him, stabbing him in the heart and slashing his hand.
Police say witnesses reported seeing the suspect get into a black Mercedes parked outside the shop.
Two other men were thought to have been in the car, which had been reported stolen several days earlier.
Shortly after the fourth anniversary of Ahmed’s death, on April 26 last year, younger brother Abubakkar Junior Jah was stabbed to death in Coolfin Road, Custom House.
It was later revealed he had been wounded in another stabbing two weeks earlier.
Police have arrested 10 people in connection with that murder investigation, which continues.
Det Ch Insp Kelly Allen said: "It's hard to imagine the pain and heartbreak Ahmed's parents have been through over the past five years after losing two of their children in such awful circumstances.
"I am certain that somebody will know who was responsible for Ahmed's death and will be able to provide the information we need to allow his family a small amount of closure.
"We know Ahmed was involved in a local gang and sadly I have to keep an open mind to this being a factor that led to this death."
The Met says while people involved in gangs may feel they cannot speak to police, any information would be treated confidentially and steps can be taken to protect the identity of anyone who comes forward.
Det Ch Insp Allen added: "A lot has changed over the past five years and I hope somebody, who felt they were unable to come forward at the time of Ahmed's death, will now have the courage to do the right thing."
Anyone with information is asked to call 020 8345 1570 or 101, quoting reference number CAD 5326/02Apr17.
To remain completely anonymous, contact the independent charity Crimestoppers via https://crimestoppers-uk.org/ or call 0800 555 111.
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