A fourth teenager has been arrested in connection with the fatal stabbing of 14-year-old Paul Erhahon.
Paul was stabbed to death on Friday night
Paul died in his mother's arms outside his home in Leytonstone, east London, on Friday night after being stabbed by a gang.
His friend was also injured in the attack and is critically ill.
Police said a 14-year-old boy was being held at an east London police station, and a further three teenagers, aged 13, 14 and 19, remained in custody.
Two of them were arrested hours after the attack while the 13-year-old was detained on Monday morning.
Paul and his friend were found in Crownfield Road, a few yards from Paul's home.
LONDON'S TEENAGE VICTIMS
James Andre Smartt-Ford, 16, shot in Streatham, 3 February
Michael Dosunmu, 15, shot in Peckham, 6 February
Billy Cox, 15, shot in Clapham, 14 February
Kodjo Yenga, 16, stabbed in Hammersmith, 14 March
Adam Regis, 15, stabbed in Plaistow, 17 March
Police believe they had been attacked in the foyer of Gean Court, a block of flats in Langthorne Road, Leytonstone.
A post-mortem examination found Paul had died from a stab wound.
Police have asked people living nearby to search their gardens for the weapon. Officers said they were investigating whether Paul's death was gang-related but were keeping an open mind.
The latest attack follows the killing of at least five other male teenagers in London in the past two months.
On Sunday churches in the area offered prayers for the boys and their families at Easter services.
Paul's aunt Jackie Mustapha revealed that Paul's mother, Ivy, was with her son as he lay dying.
Mrs Mustapha said: "His mum was there kissing him, cuddling him, trying to see if she could revive him."
Ms Mustapha also said Paul moved schools last year after he was stabbed by bullies in a "personal quarrel".
Paul went on to attend Kingsford Community School - the same school as 15-year-old Adam Regis, the nephew of athlete John Regis, who was stabbed to death last month.
Police have said they are not linking the two fatal stabbings.