Two 16 year-olds and one 17 year-old died at the scene in Hayes after the crash – leading to a CCTV appeal today by the Metropolitan Police.
An Audi A5 collided with the three boys, who police said were believed to have been on the pavement at Shepiston Lane by a bus stop at 8.40pm on Friday.
Tonight at around 7pm, a 34-year-old man voluntarily attended a north London police station. He has now been arrested on suspicion of causing death by dangerous driving and taken into custody for questioning, officials said.
A 28-year-old man who was arrested at the scene on suspicion of causing death by dangerous driving remains in police custody.
Detectives have this evening also named the victims of the fatal collision, though formal identification is yet to be completed. They were:
17-year-old Harry Louis Rice
16-year-old George Toby Wilkinson
16-year-old Josh Mcgunniess.
Detective Sergeant Michael Rapp of the Met's Serious Collision Investigation Unit said: "Earlier today, police released images of a man they wish to speak to in connection with this incident. A man has now handed himself in at a police station and been arrested before being taken into custody for questioning.
"This is a fast moving investigation and CCTV enquiries and forensic analysis of the car involved are underway.
“At least two males are believed to have left the black Audi A5 following the collision. Enquiries are ongoing to establish the full circumstances of the collision and the number of occupants in the vehicle involved.
"One thing I would also like to add is that the families who have lost loved ones have had knocks at their door from individuals purporting to be from the media.
“This has added to their trauma. I would implore all to desist from attempting to contact family members who are still barely coming to terms with their loss."
Hillingdon Borough Commander, T/Chief Superintendent Colin Wingrove said: "Our thoughts and prayers are with family and many friends of three young men who have died in such tragic circumstances.
"Tributes continue to be laid at the scene and officers will be in and around the area providing a presence and also make to themselves available to anyone who does have information and wants to speak with police.
“Detectives from the Roads and Transport Policing Command continue to lead the investigation and a number of enquiries are on going.
"There is understandably a lot of shock and grief in the local community. These three young men had their whole lives ahead of them.
"I would like to reiterate the appeal from the families that their privacy is respected at this difficult time so they can grieve in peace."
Despite the efforts of officers and London Ambulance Crew, all three boys died at the scene.
A dedicated family liaison officer continues to support the victims' families.
Anyone with information should call the Met's Serious Collision Investigation Unit on 0208 991 9555 or via Twitter @MetCC.
Alternatively, contact the independent charity Crimestoppers on 0800 555 111, or visit crimestoppers-uk.org.