Three people are still being questioned over the fatal stabbing of a 15-year-old boy in north London.
Negus McClean was attacked in Westminster Road, Edmonton, on Sunday as he tried to protect his younger brother, according to his family.
Police said they were looking for nine young men seen on mountain bikes chasing Negus before the attack.
Negus, described as intelligent and mature, was the fourth teenager to be stabbed to death in London this year.
Two men, aged 18 and 20, and a 14-year-old boy are still being questioned over the death. A 16-year-old boy, arrested at the same time, has been released on bail until next week.
The victim, who lived in Edmonton, was stabbed several times and tests showed he died from a stab wound to the heart.
Det Ch Insp Steve Clayman said officers were keen to trace a light-skinned black male, believed to be under 18, and wearing a grey or black hooded top.
The victim's 25-year-old cousin, who gave his name as Noel, said: "He was dying protecting his brother. It was a selfless act. He paid with his life.
"It was just a disgusting act of violence."