A 15-year-old boy has been chased and stabbed to death in north London.
Police found the boy in Westminster Road, Edmonton, on Sunday evening with wounds to the chest and thigh. Officers are looking for nine young men.
Negus McClean, of Edmonton, was attacked by a group of people while "selflessly" trying to protect his 12-year-old brother, a family member said. He was due to turn 16 in two weeks.
Four youths have been arrested over the fatal attack, Metropolitan Police said.
Det Ch Insp Steve Clayman said they were looking for up to nine young men who were riding mountain bikes.
He said: "He's being chased, it would appear, by seven, eight, nine young men, all on mountain bikes, it would appear as well.
"He turns up into Westminster Road, where he is actually attacked. He's stabbed a number of times."
Mr 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.
Negus is the fourth teenager to be stabbed to death in London this year, Scotland Yard said.
The victim, who has been described as "intelligent and mature", was about to celebrate his 16th birthday in two weeks' time, his relatives said.
Laying flowers at the scene, 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.
"I pray that a change happens, I pray for the guy that did this to my cousin."
Describing his cousin as someone who "loved making music" and aspired to be a rapper, he added: "He was such a mature individual, so grown up.
"His mum and dad are keeping it together.
"His brother and sister are keeping strong and needing their faith."
Another cousin said: "We just hope his death is not in vain."
The 15-year-old victim was taken to the Royal London Hospital, where he died.
Local resident Michael Athnasiou, who lives in a terraced home in Westminster Road, where the stabbing took place, said: "The gangsters are getting out of control. It is very scary."
'Lives needlessly lost'
The head teacher of Edmonton County School, which Negus attended, Dr Susan Tranter, said: "It is a tragedy that the life of a 15-year-old boy has been lost to knife crime.
"I urge all the young people of our school to shun the gang-related issues that will have contributed to this untimely death. There is no future for a young person as a member of a gang."
Councillor Doug Taylor, the leader of Enfield Council, said: "It is absolutely tragic for the whole community to have a young person killed on the streets of Edmonton.
"I can guarantee that this council will continue to work with local communities to try and ensure no more young lives are needlessly lost."
Young people have been laying flowers and paying respects at the scene and a tribute page has been set up on Facebook.
A post-mortem examination is due to take place.