Police say a 20-year-old man who died in a triple stabbing outside Parsons Green Tube station had asked a drug dealer to move on before he was killed.
Omid Saidy, from Fulham, was pronounced dead at the scene at 8.30pm on Monday, while two males, aged 20 and 16, were taken to hospital.
Scotland Yard said he was killed after he confronted a drug dealer and another male who was with him.
Detectives believe Mr Saidy chased the pair and was fatally stabbed when he caught up to them.
Detective Chief Inspector Noel McHugh, from the Met, said: "A young man has tragically lost his life for simply asking a drug dealer to move on. I urge anyone who can assist our investigation to come forward without delay."
The victim's 20-year-old friend, who arrived on the scene on a moped, was also stabbed when he came to his aid, police said.
The bike collided with a parked vehicle at the scene, but police said the incident is not a "moped-enabled crime."
One of the suspects, described as a black male dressed in dark clothing, fled down Harbled Road in the direction of Fulham Court.
A 16-year-old white male, who was discharged from hospital after he was treated for his injuries, has been arrested on suspicion of murder and attempted murder.
Appealing for witnesses, Mr McHugh added: "It is imperative that we speak to anyone who was in the area of Parsons Green station between 7pm and 8pm who may have witnessed the murder or the actions of the suspects immediately following the incident.
"In addition, I am keen to identify the black male who fled Parsons Green Lane immediately following the murder and am appealing for footage that members of the public may have recorded as events in the area took place."