An "innocent bystander" killed in a convenience store died of a single gun shot wound to the back, Scotland Yard said.
Ryan Bravo, 18, died after he was caught up in an apparent gang fight at a Costcutters in Walworth, south east London.
A 19-year-old man arrested shortly after the shooting was being questioned at a south London police station.
A post-mortem examination carried out at Greenwich Mortuary revealed the cause of death.
The victim's mother, Elfrida Kiffin, said he was a loving brother, partner and son who was already dearly missed.
In a statement, she said: "I would like to state that Ryan was a decent, hard-working young man and loving son. This is an extremely difficult time for all of us that knew and loved Ryan and I would ask that we are given the time and space to grieve in private.
"I would urge anyone that knows or saw anything to come forward and assist officers from Operation Trident in their ongoing efforts to trace witnesses to this cold-blooded and reckless act."
Police believe the intended victims of the shooting could hold the key to identifying the gunman and his accomplices.
Detectives were hunting two young black men who rushed inside the Costcutter shop in a bid to escape the armed moped-riding gang. As they ducked for cover, at least two shots were fired towards the shop, fatally wounding Mr Bravo.
The young black man, who became the 22nd teenager to die violently in the capital this year, had dropped in to buy snacks with his brother and cousin.
