A 15-year-old boy has been killed in a double stabbing outside a branch of Sainsbury's.
Patrolling Met Police officers came across a fight outside the supermarket in Wandsworth, south-west London, just before 17:00 GMT on Thursday.
Another 15-year-old boy was taken to hospital with injuries which are not thought to be life-threatening.
A 17-year-old has been arrested on suspicion of affray and a 19-year-old on suspicion of causing GBH.
Det Ch Insp Vicky Tunstall said: "This was a busy area, very near to the shopping centre.
"There will be people who saw or heard something suspicious that could help us to piece together the final moments for the victim and possibly identify who was responsible.
"It is these people we urgently need to speak to."
Detectives said the stabbing on Garratt Lane was not being treated as terror-related and officers are confident they know who the victim is.
His next of kin have been informed and a post-mortem examination will be held.
Det Supt Owain Richards described the stabbings as "shocking".
"I know that the detectives on this case will be working tirelessly to understand what led up to this tragic incident and ultimately to bring the person or people responsible to justice," he said.
The force previously said a third teenager had been injured but later released a correction saying only two 15-year-old boys had been stabbed.