A man has been taken to hospital and is in a critical condition after a car overturned in the early hours of Saturday.
A Mercedes-Benz A Class overturned in Clwyd Road near its junction with Penderry Road at 4.07am in Penlan, Swansea, a police spokesman said.
A 21-year-old man suffered serious injuries and was taken to the University Hospital of Wales in Cardiff where his condition is described as critical.
Officers investigating the incident are keen to identify a second man who is also believed to have been in the car but left the scene before officers arrived.
The road remains closed and is expected to remain so throughout the morning.
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A spokesman for South Wales Police said on Saturday: "We are investigating a serious road traffic collision which happened earlier today in Penlan.
"A green Mercedes-Benz A Class overturned on Clwyd Road near its junction with Penderry Road at 4.07am.
"A 21-year-old man sustained serious injuries and was taken to the University Hospital of Wales, Cardiff, where his condition is described as critical.
"Officers are keen to identify a second man who is also believed to have been in the car but left the scene prior to their attendance.
"Members of the public are thanked for their patience,while investigation work is carried out at the scene and for their patience during the road closures."
Officers are keen to talk to anyone who witnessed the incident or the manner in which the green Mercedes-Benz was being driven before the incident happened.
Anyone with any information is asked to call 101 and quote occurrence number *360836.
