Three teenage boys were killed when a car hit them near a bus stop in west London on Friday evening in what police have called a "horrific" crash.
Two of the boys, who were aged 16 and 17 and had been heading to a birthday party, have been named locally as Harry Louis Rice and George Wilkinson.
The crash happened in Shepiston Lane, Hayes, near a petrol station.
A man, 28, who was in the Audi involved in the crash, was arrested on suspicion of causing death by dangerous driving.
The suspect was detained by members of the public, and was then taken to hospital for treatment before going to a north London police station for questioning.
At least one other male is reported to have left the car following the collision.
Police said they were doing everything they could to establish the full facts and trace everybody involved.
The boys' next of kin have been informed, and a post-mortem examination will be held in due course.
The victims have yet to be formally identified, the Met Police said.
Other teenagers who were standing with the boys were uninjured.
Friends who left flowers at the scene on Saturday said the teenagers were going to a 16th birthday party.
Staff at the nearby Esso garage said a group had been fighting at the entrance to the forecourt just before the crash.
It is believed the Audi was travelling at high speed before it mounted the pavement just outside the entrance and travelled a few yards down the road before hitting a lamp post.
One person who passed the crash scene said he saw friends of the victims help a fast-response paramedic get equipment from the car.
Amamdeep Matharoo, 27, said: "The lads were shouting on the phone. They were saying 'three of our friends are dead'. They were hysterical and in pieces."
Tony Kitchin, 44, who works as a security guard at a haulage firm nearby, said: "This road is busy 24/7. It's a speed trap.
"I'm really surprised we haven't had more horrific accidents like this.
"No parent should have to deal with that. My heartfelt sympathies go out to the family and friends of the people who lost their lives last night."
Det Sgt Jane Shaw, from the Met's serious collision investigation unit, said: "There are horrific circumstances that have resulted in three young people losing their lives."
She appealed for anyone who saw what happened to contact the police.
Temporary road closures have now been lifted.