A collision in Hayes which left three teenage boys dead involved an Audi that was travelling at "high speed", police said.
Detectives revealed that there was a "large party" near the scene on Friday night (January 26) and witnesses are asked to come forward "immediately".
The three boys - two aged 16 and one aged 17 - are understood to have been on the pavement with a group of their friends on Shepiston Lane at around 8.41pm when a black Audi A5 collided with them.
Police were called and arrived shortly after to Shepiston Lane, Hayes, on the M4 Junction 4, near the Esso petrol station along with London Ambulance Service (LAS).
DS Jane Shaw, of the Met's Serious Collision Investigation Unit, said: "These are horrific circumstances that have resulted in three young people on a night out losing their lives.
"Dedicated family liaison officers have been put in place to support the next-of-kin of the victims.
"Our thoughts are with them.
"We know that there was a large party nearby and many people would have been travelling through the area and saw what happened or witnessed the aftermath.
"I need these witnesses to contact police immediately.
"We are also particularly keen to trace other motorists travelling through the area at the time."
Despite the best efforts of officers and LAS, all three schoolboys died at the scene.
Mourners gathered at the scene in Shepiston Lane to pay their respects and lay flowers the day after the tragedy .
The other teenagers present were not injured, police confirmed.
All of the next of kin of the deceased have been informed and a post-mortem examination will be held in due course.
The victims have yet to be formally identified.
Metropolitan Police said on Saturday (January 27) that the vehicle involved is believed to have been travelling at "high speed", and came to a stop a short distance away from the collision.
A spokesman for the force said: "At least two males are reported to have left this car following the collision.
"One of them was detained by members of the public.
"This 28-year-old male was arrested on suspicion of causing death by dangerous driving.
"He was taken to a west London hospital for treatment and has now been taken to a north London police station for questioning.
"Enquiries are ongoing to establish the full circumstances of the collision and the number of occupants in the vehicle involved."
Temporary road closures have now been lifted.
Officers from the Met's Roads and Transport Policing Command continue to investigate.
Anyone who witnessed the collision is asked to call the Serious Collision Investigation Unit directly on 0208 991 9555 quoting ref 7843 of 26 January, or call 101 or tweet @MetCC.
Alternatively, contact the independent charity Crimestoppers on 0800 555 111, or visit crimestoppers-uk.org.
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