A teenager who was fatally stabbed in a council headquarters in west London was attending a knife awareness course.
Hakim Sillah was attacked in the youth offending service department at the Hillingdon Civic Centre in Uxbridge on Thursday.
The Met said a group had gathered at the venue when a fight broke out. The 18-year-old was taken to hospital but died an hour later.
A 17-year-old boy who was arrested on suspicion of murder remains in custody.
A teenage boy who tried to stop the fight also sustained a knife wound to his ear.
He was praised by detectives for "bravely" trying to break up the fight.
In a statement, Mr Sillah's family described him as "a lovely lad who cared about his family".
"He loved looking after his little brother, who had been ill," they said.
Det Ch Insp Noel McHugh described the attack as "an absolute tragedy" and praised the teenager who tried to stop the fight.
"A young man with his whole life ahead of him has been fatally attacked and his family are absolutely devastated," he said.
"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."
Hillingdon Council said it was "offering support and counselling" to any of its employees affected.
It is the second murder investigation to be launched in Hillingdon this year - after Tashan Daniel was stabbed to death at Hillingdon Tube Station on 24 September.
Mr Daniel's family plans to organise a march in central London on 7 December to raise awareness of the impact of knife crime.