A 15-year-old boy who died when the car he was travelling in hit a tree has been named by police as Matthew Thomas from the Portmead area of Swansea.
The wreckage of the car at Cockett Road, Swansea
A 17-year-girl also in the vehicle remains critically ill at the city's Morriston Hospital.
They were among five teenagers - three males and two females aged between 15 and 19 - in a Peugeot 106 which crashed at Cockett Road early on Thursday.
South Wales Police said no other vehicle was involved in the incident.
The Peugeot was found by a police patrol car at 0320 BST.
Mid and West Wales Fire and Rescue Service said firefighters released four people from the car and another was out before they arrived at the scene.
The male driver, 19, suffered facial and head injuries while an 18-year-old female passenger and a 17-year-old boy both suffered chest injuries.
They are said by police to be recovering from their injuries.
Supt Phil Davies appealed to anyone who witnessed the collision or who may have seen the red Peugeot 106 being driven in Swansea before the collision to come forward.
"Although the investigation is in its early stages we are satisfied that no other vehicle was involved," he said.
There were long tail backs during the morning rush hour as police closed Cockett Road between the junction with Cwmbach Road and St Peter's Terrace.
Witnesses should call police on 01792 456999,or Crimestoppers on 0800 555111.