A 16-year-old boy was found stabbed to death in a stairwell at a block of flats in south-west London after a party, police said.
The boy was found on the ground floor of the block in Wandsworth at 1.30am on Sunday, Scotland Yard said. An undisclosed number of people have been arrested.
Detective Chief Inspector Nick Scola, of the Metropolitan police's homicide and serious crime command, said he wanted to speak to a group of between five and eight men seen running from the scene at Albon House, in Neville Gill Close.
"I believe the victim had attended a party in Albon House," Scola said. "I need to trace everyone at that party, and in particular I am appealing for any information about a group of between five and eight males seen running from the scene into the surrounding streets and possibly through King George's Park."
The boy's next of kin have been informed. Those arrested are in custody at police stations in south and central London.
Police cordoned off a stretch of road outside Albon House as forensics teams examined the scene.