Two 14-year-old boys are being questioned by police in Newham, East London, after a 15-year-old boy was found stabbed to death outside a youth centre.
The boy, who was of Somali origin, was taken to hospital, but was dead on arrival.
It's thought he was killed after an argument over a £10 debt.
A man and a woman were also arrested and questioned at an East London police station.
Police are now asking for any witnesses to come forward. They have already found two knives after a search of the area, but it was too soon to say whether one was the murder weapon.
Police said if anyone has any information about the attack, they should call Edmonton police station on 020 8345 4363.