This is the first picture of fatal Uxbridge stab victim Hakim Sillah.
The Met tonight released the image as it continued its appeal for witnesses and information.
The 18-year-old was stabbed at the youth offending service in Hillingdon Civic Centre at about 4.30pm on Thursday afternoon.
Mr Sillah had been attending an “awareness event”. He died of his injuries about an hour later in hospital.
A 17-year-old boy was arrested on suspicion of murder and was still being questioned by police on Friday.
Det Ch Insp Noel McHugh, who is leading the investigation, said: “This is an absolute tragedy. A young man with his whole life ahead of him has been fatally attacked and his family are absolutely devastated.
"We are doing everything we can to find those involved. What we know so far is that a fight broke out between males at the location and as a result this young man received fatal injuries. A second independent male bravely tried to intervene to break up the fight and as a result was also stabbed.”
His injuries were not life-threatening.
Det Ch Insp McHugh went on: “We want to hear from anyone who was there so we can understand what happened prior to the stabbing taking place and events immediately afterwards. One male in currently in custody and is being interviewed.
“Whilst we have made an arrest, and we are not seeking any other suspects, we urge anyone who has information on what took place to come forward. Do the right thing and call police with what you heard and saw.”
Anyone with information is asked to call police on 020 8785 8099 or Crimestoppers anonymously on 0800 555 111.
