Police say they would like to speak to teenager Mahamed Barre in connection with the murder of Jamil Palmer who was stabbed to death in Feltham.
Palmer, 18, of Hampton, Middlesex, was stabbed after walking through Crane Park in Feltham, Hounslow, on 7 May and found slumped in the road near Evans Grove.
Barre, 17, is alleged to have suffered a head injury in an "altercation" with Palmer earlier on the day of the murder.
Police say Barre is a Somali of slim build.
Scotland Yard is also offering a reward of up to £15,000 for information leading to the arrest and prosecution of the person or people responsible for the killing.
Detective chief inspector Noel McHugh from the Homicide and Major Crime Command, leading the investigation, said: "I am seeking help from the local community to help locate Mahamed Barre. I would ask anyone who knows of his whereabouts, or has any information that could assist us, to come forward."
McHugh added: "I also appeal directly to Mahamed Barre to present himself at a police station so this matter can be dealt with."
Four boys have been charged with Palmer's murder. These include a 16-year-old from Redding, a 15-year-old from North London, a 15-year-old and a 16-year-old from West London.
Anyone with information is asked to contact the incident room on 0208 785 8099 or call Crimestoppers anonymously on 0800 555 111. Anyone who sees Mahamed Barre should call 999.