A youth worker stabbed to death after he confronted a suspected drug dealer died after "trying to do the right thing", police said.
Omid Saidy, 20, was attacked outside Parsons Green Tube station in west London on Monday evening.
Detectives believe he was killed after he confronted a suspected drug dealer and another teenager at about 7.30pm outside a Co-op store.
He chased them down Parsons Green Lane, but was stabbed outside a dry cleaners near the station.
Police are looking for a black male, aged in his late teens who was wearing dark clothing, who fled the area immediately after the stabbing.