A 17-year-old male was arrested on suspicion of murder on Wednesday, 14 April in relation to the fatal stabbing of Levi Ernest-Morrison, 17, in Sydenham Road, SE26 on Saturday, 10 April.
The teenager remains in custody at a south London police station and enquiries into the circumstances continue.
Homicide detectives from Specialist Crime continue to investigate.
Detective Chief Inspector Chris Wood, leading the investigation, said: “I am appealing for anyone who has information concerning the identity and whereabouts of the people involved in this murder to share what they know with my team.
“It is often the case that friends or acquaintances of the victim know who was responsible, if you are one of those people, please do the right thing by your friend and get in touch with us.
“If you don’t want to speak directly to police you can pass your information to Crimestoppers who will not even ask you for your name, you can do this by telephone or online. Or, if you prefer, you can speak directly to us where all information will be treated in the strictest confidence. Whatever you decide, please consider Levi and his devastated family and do the right thing.”
A 36-year-old woman was arrested on Sunday, 11 April. She was charged with murder on Tuesday, 13 April and appears in custody at the Old Bailey on Friday, 16 April.
Anyone who has information and wants to speak with police should call 101 or Tweet @MetCC quoting 6072/10APR.