A 15-year-old boy has been arrested on suspicion of murder after a teenager was stabbed inside a council building close to Wembley Stadium.
The suspect was arrested on Tuesday and remains in police custody.
Emergency services rushed to Brent Civic Centre in Engineers Way, Wembley, north-west London, following reports of a stabbing.
London Ambulance Service confirmed air ambulance crews attended the scene and found a 17-year-old boy with stab wounds.
Despite the best efforts of paramedics, the teenager was confirmed to have died.
Formal identification has yet to take place, but the victim is believed to be David Wynter, 17, Met Police said.
His next of kin are being supported by specially trained officers.
The centre, which was reportedly evacuated amid the incident, is home to Brent Council.
A 15-year-old boy has been arrested on suspicion of murder after a teenager, 17, was stabbed inside a council building close to Wembley Stadium
Police officers at the scene of the stabbing at Brent Civic Centre in Wembley on Tuesday
A witness said they saw someone lying on the floor of the library in the building, and spatters of blood.
The Met said a Section 60 order has been authorised, granting officers enhanced powers to stop and search people and vehicles.
This applies to the Wembley Park, Barnhill, Tokyngton, Stonebridge, Roundwood, Dollis Hill, Willesden and Welsh Harp ward areas of Brent.
The order runs from 3pm today until 3.30am on August 27.
Superintendent Chris Clarke, lead for emergency response policing in the area, said: 'This was a tragic incident, and we are aware of the significant concern the stabbing has caused in the local community.
'A police presence remains in and around the crime scene.
'We would like to reiterate that officers do not believe there is any wider risk to the community.'
Councillor Muhammed Butt, Leader of Brent Council, said in a statement after the incident: 'Today, Brent has been shaken by an appalling and deeply tragic incident at the Civic Centre, in which a person has lost their life.
'Our thoughts are first and foremost with the victim, their family and loved ones, as well as the staff, residents and visitors who witnessed what happened.
'The Civic Centre has been evacuated and will remain closed until further notice. I would urge people to avoid the area and allow the police and emergency services to carry out their work.
'There is an active police investigation underway - please do not speculate or repeat any unverified claims online. We are working closely with the Metropolitan Police and will provide further updates as soon as we are able.
'I want to express my sincere thanks to the police, paramedics and emergency services for their swift response, and to our staff for the professionalism, compassion and calm they have shown today in the most difficult of circumstances.'
Anybody with information is urged to call 101, quoting reference 4032/25AUG.