Five teenagers are being held on suspicion of dangerous driving after four people were killed in a collision between two cars.
Three 17-year-olds and a 68-year-old woman died and two others are in hospital following the crash in Powys, Mid Wales, just before 10.15pm on Friday.
The 68-year-old has been named by police as Margaret Elizabeth Challis, from the Merthyr area.
The teenagers who were killed – two males and a female – were all from Barry. They have not been named.
The crash happened as a number of vehicles travelled in a convoy on the A470 near Storey Arms, Brecon, between Libanus and RAC bends.
The two cars involved in the collision were a black Volkswagen Golf estate and a green Volkswagen Golf.
The teenagers were in the green Golf and Mrs Challis was a passenger in the black Golf estate, which was not believed to be part of the convoy.
Five male drivers, aged 17 and 18, who were in other vehicles, have been arrested, Dyfed Powys Police said.
The driver and front-seat passenger in the black Golf estate have been taken to hospital. The driver’s condition is described as serious.
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.”
Anyone who witnessed the crash is asked to contact Dyfed Powys Police on 101.