An 18-year-old man has been stabbed to death in a west London council's central offices.
The victim was stabbed in the chest in Hillingdon Civic Centre on High Street, Uxbridge, at about 16:40 GMT.
He was taken to hospital but pronounced dead less than an hour later. Another teenage boy was also stabbed during the attack.
A 17-year-old boy has been arrested on suspicion of murder, Scotland Yard said.
The other injured teenager suffered a knife injury to his ear, but his injuries were not considered life-threatening.
The Met has granted itself enhanced stop-and-search powers, under section 60 of the Criminal Justice and Public Order Act.
This allows officers to search anyone in a designated area without "reasonable grounds".
Det Ch Insp Noel McHugh described the stabbing as "a tragic loss of life".