A third person was arrested over the murder of Hampton teenager Jamil Palmer on Tuesday night.
The boy, believed to be 15, was arrested on suspicion of murder at a west London police station and currently remains in custody.
The arrest came one week to the day Mr Palmer was killed in broad daylight near Crane Park in Feltham.
Last week, an 18-year-old man and 16-year-old boy were arrested on suspicion of murder and released on bail, due to return to police in early June.
Yesterday detectives called for witnesses to abandon "misguided loyalty" to help bring justice to former Hampton Junior School, Teddington School, Richmond College and Kingston College student Mr Palmer.
Detective Chief Inspector Noel McHugh, of the homicide and major crime command, said: "Jamil was attacked in Crane Park shortly before 3.30pm and the park would have been busy with people.
"Some witnesses have come forward to assist us with our investigation but I am certain there are a number of other people who have yet to speak to us.
"It is clear that there is a misguided loyalty where some people are not speaking to the police, protecting those involved.
"I would ask those who are reluctant to come forward to do so, so Jamil and his family can get the justice they deserve."
Anyone with information should call the incident room on 020 8785 8099 or call Crimestoppers anonymously on 0800 555 111.