A second person has been arrested on suspicion of murder following the stabbing of a 17-year-old girl in an east London park.
Jodie Chesney was attacked while playing music with friends in Harold Hill, Romford, on 1 March.
The Met said the male suspect was being held in custody in London.
On Thursday evening, hundreds of people marched through the town centre to protest against the killing.
A 20-year-old man who was arrested in Leicester on 5 March on suspicion of murder also remains in custody.
Det Ch Insp Dave Whellams said there was still “no clear motive” for the attack, which he described as “very unusual”.
“We retain an open mind and can’t rule anything out,” he added.
Jodie was with five other teenagers near a children’s playground in Harold Hill at about 21:00 GMT when they became aware of two males who they did not speak to.
The pair then left but about 30 minutes later they returned and walked straight towards the group where one of them stabbed the victim in the back.
A post-mortem examination gave the cause of the Scout’s death as trauma and haemorrhage.
Numerous purple ribbons – Jodie’s favourite colour – have been hung across Harold Hill and Romford.
Students and teachers at Havering Sixth Form College, where she studied, are also holding a “Purple Friday” to remember the 17-year-old with people being invited to dress in purple.
The 17-year-old’s father said she lost “so much blood” in the “ferocious attack” and that clearly “someone meant to murder her”.
Describing his daughter as a “proud geek” and a “great girl”, Peter Chesney called on anyone who knew the killer to “just dob them in, grass them up.”
Jodie was the fifth teenager to be stabbed to death in London this year.