Three people have been arrested on suspicion of murdering schoolboy Alan Cartwright, who was stabbed to death while riding a bicycle with his friends.
An 18-year-old man was arrested in the early hours of this morning after attending a north London police station, Scotland Yard said.
A 17-year-old boy and a 21-year-old man were also arrested in Camden, north London, this morning and remain in custody.
The arrests follow an emotional appeal by Alan's family for witnesses to come forward to help track down the 15-year-old's killers.
Police released "shocking" footage of the moment the schoolboy was stabbed in the chest as he cycled with friends along a busy road in Islington, north London.
He managed to continue riding a short distance after the attack in Caledonian Road on Friday, but soon collapsed and was later pronounced dead at the scene.
A second suspect is shown lunging at one of the teenager's friends and knocking him off his bike, while a third crosses the road and grabs another friend to pull him off his bicycle.
A post-mortem examination found that Alan died from a single stab wound to the chest.
His mother and sister described the Army cadet and Celtic and Arsenal fan as a "joker" who loved bikes and never caused any trouble.
Fighting back tears during an appeal yesterday, Alan's mother, Michelle Watson, 39, said: "He always made you laugh about anything and everything. He was just the funny one out of his friends.
"I feel angry, I want somebody found for this. I just feel numb."