A teenager was shot when a row on a Tube train spilled out on to a station platform in east London.
At first it was thought the victim had been stabbed, however in hospital a bullet was found in his abdomen.
The shooting happened following an argument on-board a Hammersmith and City Line train out of London on Wednesday night.
The four got off the train at Barking Tube station where the victim was shot, police said.
Passenger Charlotte Maher witnessed the attack and tweeted at the time: "Passenger alarm pulled and a brave gentleman tripped the individual with the knife [sic] up.
"Suspect appeared to flee along the platform after the doors were opened, the injured individual also fled," she said, adding that she "saw blood up the stairs" as she left the station.
Det Ch Insp Sam Blackburn said: "This was a violent and rare attack which took place on a late-night train out of London.
"Officers were on scene within minutes and we are working hard to identify and trace the men responsible for this assault."
British Transport Police (BTP) believe the victim was arguing with another two other suspects who fled the scene after the shooting.
No arrests have been made at this stage.