Five teenagers have been arrested on suspicion of dangerous driving following a car crash near Brecon, Powys, on Friday 6 March in which four people died: three youths aged 17, and 68-year-old Margaret Elizabeth Challis.
The crash happened on the notorious A470 near Storey Arms, Brecon, when a number of vehicles were reportedly travelling in a convoy.
One of the cars, a green Volkswagen Golf, collided with a black Volkswagen Golf estate in which Mrs Challis, from the Merthyr area, was a passenger.
Two males in the green Golf died at the scene. A female died later in hospital, as did Mrs Challis.
Two others in Mrs Challis's car – a driver and front-seat passenger – were injured, the driver seriously. All three teenagers were from the Barry, Vale of Glamorgan area.
Among those killed were students and former students of Ysgol Bro Morgannwg comprehensive school in Barry. A statement on the school website said: "Following a car accident we received some sad news about our pupils. Our thoughts and sympathies are with their families and we wish a speedy recovery to all those who have also been injured in the incident."
Five male drivers, all 17 or 18, who were driving other vehicles have been arrested. Dyfed Powys police are asking for anyone who witnessed the collision to call 101.
Superintendent Chris Curtis said: "This is a tragic incident and our thoughts are with the families at this difficult time. We are working with South Wales Police in order to provide support to the families affected and also the communities they live within. We will work with partners to ensure that the incident is fully investigated."
The A470 is often used as an unofficial race track, with footage posted on YouTube.
Local businessman Jason Griffiths told The Telegraph: "I have heard they were racing over the mountain road – the crash happened at the first straight where you would get a chance to overtake. One of the cars seems to have crossed the road and may have hit a dry stone wall opposite.
"Youngsters do speed in the road – you see them bumper to bumper sometimes. It is another tragedy – there are always flowers laid on the roadside. Now there will be more."
The road remained closed through Friday evening, reopening at 10am on Saturday (7 March).