A toddler in a pushchair has been killed and a man in his 30s has been critically injured in a crash in north-west London.
Another pedestrian was also struck by the driver of a car in Eastcote Road, Ruislip, on Sunday afternoon, the Metropolitan Police said.
A man in his 40 has been arrested on suspicion of causing death by dangerous driving.
He is also suspected of driving while unfit through drugs.
Det Sgt Sarah Donegan said: "Our thoughts are with the child's parents at what must be an unimaginably difficult time.
"We have launched an investigation to establish the circumstances of what happened and I am asking anyone who was in the vicinity at that time who may have seen something, or has dashcam footage or CCTV, to come forward and help us with our investigation."